|P-51 Mustangs of the 375th Fighter Squadron, 361st Fighter Group, summer 1944. The aircraft second from the camera has the recently introduced dorsal fin.|
|National origin||United States|
|Manufacturer||North American Aviation|
|First flight||26 October 1940|
|Status||Retired from military service 1984, still in civil use|
|Primary users||United States Army Air Forces
Royal Air Force, numerous others (see below)
|Unit cost||US$50,985 in 1945 ($622,225 in current value)|
|Variants||North American A-36
North American Mustang Mk.X
|Developed into||F-82 Twin Mustang
Piper PA-48 Enforcer
The North American Aviation P-51 Mustang was an American long-range single-seat World War II fighter aircraft. Designed and built in just 117 days, the Mustang first flew in Royal Air Force (RAF) service as a fighter-bomber and reconnaissance aircraft before conversion to a bomber escort, employed in raids over Germany, helping ensure Allied air superiority from early 1944. The P-51 was in service with Allied air forces in Europe and also saw limited service against the Japanese in the Pacific War. The Mustang began the Korean War as the United Nations' main fighter, but was relegated to a ground attack role when superseded by jet fighters early in the conflict. Nevertheless, it remained in service with some air forces until the early 1980s.
As well as being economical to produce, the Mustang was a fast, well-made, and highly durable aircraft. The definitive version, the P-51D, was powered by the Packard V-1650, a two-stage two-speed supercharged version of the legendary Rolls-Royce Merlin engine, and was armed with six .50 caliber (12.7 mm) M2 Browning machine guns.
After World War II and the Korean War, many Mustangs were converted for civilian use, especially air racing. The Mustang's reputation was such that, in the mid-1960s, Ford Motor Company's Designer John Najjar proposed a new youth-oriented coupe automobile be named after the fighter.[nb 1]
In 1939, shortly after World War II began, the British government established a purchasing commission in the United States, headed by Sir Henry Self. Serving along with Sir Wilfrid Freeman, the "Air Member for Development and Production," in 1938, Self was given overall responsibility for Royal Air Force (RAF) production and research and development. He sat on the (British) Air Council Sub-committee on Supply (or "Supply Committee") and one of his many tasks was to organize the manufacture of American fighter aircraft for the RAF. At the time, the choice was very limited; none of the U.S. aircraft already flying met European standards, with only the Curtiss P-40 Tomahawk coming close. The Curtiss-Wright plant was running at capacity, so even that aircraft was in short supply.
North American Aviation (NAA) was already supplying their Harvard trainer to the RAF, but were otherwise underutilized. NAA President "Dutch" Kindelberger approached Self to sell a new medium bomber, the B-25 Mitchell. Instead, Self asked if NAA could manufacture the Tomahawk under license from Curtiss.
Kindelberger replied that NAA could have a better aircraft with the same engine in the air in less time than it would take to set up a production line for the P-40. The Commission set as conditions the fighter be armed with four .303 machine guns, be equipped with the Allison V-1710 liquid-cooled engine, cost no more than $40,000, and stipulated the first production aircraft should be received by January 1941.
The result of the MAP order was the NA-73X project (from March 1940). The design followed the best conventional practice of the era, but included two new features. One was a new NACA-designed laminar flow wing, which was associated with very low drag at high speeds. The other was the use of a new radiator design (one Curtiss had been unable to make work) that used the heated air exiting the radiator as a form of jet thrust in what is referred to as the "Meredith Effect". Because NAA lacked a suitable wind tunnel, it used the GALCIT 10 ft (3.0 m) wind tunnel at Caltech. This led to some controversy over whether the Mustang's cooling system aerodynamics were developed by NAA's engineer Edgar Schmued or by Curtiss, although historians and researchers dismiss the allegation of stolen technology; such claims are likely moot in any event, as NAA had purchased the complete set of P-40 and XP-46 wind tunnel data and flight test reports for US$56,000.
While the United States Army Air Corps could block any sales it considered detrimental or not in the interest of the United States, the NA-73 represented a special case. In order to ensure deliveries were uninterrupted, then-Colonel Oliver P. Echols arranged with the Anglo-French Purchasing Commission to have the RAF get its aircraft in exchange for NAA providing two free examples to the USAAC for evaluation.
The initial placing of the contract was on 24 April. By then the executive head of the British Ministry of Aircraft Production (MAP), Freeman ordered 320 aircraft in March 1940. In September, MAP increased the production order by 300. The prototype NA-73X was rolled in early August, just 117 days after the order was placed, and first flew on 26 October 1940, just 178 days after the order had been placed—an uncommonly short gestation period. In general, the prototype handled well and the internal arrangement allowed for an impressive fuel load. The aircraft’s two-section, semi-monocoque fuselage was constructed entirely of aluminum alloy to save weight. It was armed with four .30 in (7.62 mm) M1919 Browning machine guns in the wings and four .50 in (12.7 mm) M2 Browning machine guns: two in the wings and another two mounted under the engine and firing through the propeller arc using gun synchronizing gear.
It was quickly evident that the Mustang's performance, although exceptional up to 15,000 ft (4,600 m), was markedly reduced at higher altitudes. The single-speed, single-stage supercharger fitted to the Allison V-1710 engine had been designed to produce its maximum power at a low altitude. Above 15,000 feet, the supercharger's critical altitude rating, the power dropped off rapidly.
Prior to the Mustang project, the USAAC had Allison concentrate primarily on turbochargers in concert with General Electric; the turbochargers proved to be reliable and capable of providing significant power increases in the P-38 Lightning and other high-altitude aircraft, in particular in the Air Corps's four-engine bombers. Most of the other uses for the Allison were for low-altitude designs, where a simpler supercharger would suffice. Fitting a turbocharger into the Mustang proved impractical, and Allison was forced to use the only supercharger that was available.
In spite of this, the Mustang's advanced aerodynamics showed to advantage, as the Mustang Mk I was about 30 mph (48 km/h) faster than contemporary Curtiss P-40 fighters using the same engine (the V-1710-39 producing 1,220 hp (910 kW) at 10,500 ft (3,200 m), driving a 10 ft 6 in (3.20 m) diameter, three-blade Curtiss-Electric propeller). The Mustang Mk I was 30 mph (48 km/h) faster than the Spitfire Mk VC at 5,000 ft (1,500 m) and 35 mph (56 km/h) faster at 15,000 ft (4,600 m), despite the British aircraft's more powerful engine.
The first production contract was awarded by the British for 320 NA-73 fighters, named Mustang Mk I by the British (the name being selected by an anonymous member of the British Purchasing Commission). Two aircraft of this lot delivered to the USAAC for evaluation were designated XP-51.[nb 2] About 20 Mustang Mk Is were delivered to the RAF, making their combat debut on 10 May 1942. With their long range and excellent low-altitude performance, they were employed effectively for tactical reconnaissance and ground-attack duties over the English Channel, but were thought to be of limited value as fighters due to their poor performance above 15,000 ft (4,600 m).
A second British contract called for 300 more (NA-83) Mustang Mk I fighters. In September 1940, 150 aircraft, designated NA-91 by North American, were ordered under the Lend-Lease program. These were designated by the USAAF as P-51 and initially named Apache, although this was soon dropped, and the RAF name, Mustang, adopted instead. The British designated this model as Mustang Mk IA. The Mustang Mk IA was identical to the Mustang Mk I, except that the wing-mounted machine guns were removed and replaced with four long-barrelled 20 mm (.79 in) Hispano Mk II cannon.
A number of aircraft from this lot were fitted out by the USAAF as F-6A photo-reconnaissance aircraft. The British would fit a number of Mustang Mk Is with similar equipment. Also, two aircraft of this lot were fitted with Packard-built Merlin engines. These were identified as the Model NA-101 by North American and XP-78 by the USAAF, later redesignated XP-51B.
On 23 June 1942, a contract was placed for 1,200 P-51As (NA-99s), later reduced to 310 aircraft. The P-51A was the first version to be procured as a fighter by the USAAF and used a new Allison V-1710-81 engine, a development of the V-1710-39, driving a 10 ft 9 in (3.28 m) diameter three-bladed Curtiss-Electric propeller. The armament was changed to four wing-mounted .50 in (12.7 mm) Browning machine guns, two in each wing, with a maximum of 350 rounds per gun (rpg) for the inboard guns and 280 rpg for the outboard. Other improvements were made in parallel with the A-36, including an improved, fixed air duct inlet replacing the movable fitting of previous Mustang models and the fitting of wing racks able to carry either 75 or 150 U.S. gal (284 or 568 l) drop tanks, increasing the maximum ferry range to 2,740 mi (4,410 km) with the 150 gal (568 l) tanks. The top speed was raised to 409 mph (658 km/h) at 10,000 ft (3,000 m). A total of 50 aircraft were shipped to England, serving as Mustang Mk IIs in the RAF.
On 16 April 1942, Fighter Project Officer Benjamin S. Kelsey ordered 500 A-36 Apaches, a redesign that included six .50 in (12.7 mm) M2 Browning machine guns, dive brakes, and the ability to carry two 500 lb (230 kg) bombs. Kelsey would rather have bought more fighters but was willing instead to initiate a higher level of Mustang production at North American by using USAAC funds earmarked for ground-attack aircraft.
The 500 were designated A-36A (NA-97). This model became the first USAAF Mustang to see combat. One aircraft was passed to the British who gave it the name Mustang Mk I (Dive Bomber).
In April 1942, the RAF's Air Fighting Development Unit (AFDU) tested the Mustang and found its performance inadequate at higher altitudes. As such, it was to be used to replace the Tomahawk in Army Cooperation Command squadrons, but the commanding officer was so impressed with its maneuverability and low-altitude speeds that he invited Ronnie Harker from Rolls-Royce's Flight Test establishment to fly it. Rolls-Royce engineers rapidly realized that equipping the Mustang with a Merlin 61 engine with its two-speed two-stage supercharger would substantially improve performance and started converting five aircraft as the Mustang Mk X. Apart from the engine installation, which utilized custom-built engine bearers designed by Rolls-Royce and a standard 10 ft 9 in (3.28 m) diameter, four-bladed Rotol propeller from a Spitfire Mk IX , the Mustang Mk X was a straightforward adaptation of the Mustang Mk I airframe, keeping the same radiator duct design. The Vice-Chief of the Air Staff, Air Marshal Sir Wilfrid R. Freeman, lobbied vociferously for Merlin-powered Mustangs, insisting two of the five experimental Mustang Mk Xs be handed over to Carl Spaatz for trials and evaluation by the U.S. 8th Air Force in Britain.
The high-altitude performance improvement was remarkable: the Mustang Mk X (serial number AM208) reached 433 mph (697 km/h) at 22,000 ft (6,700 m), and AL975 tested at an absolute ceiling of 40,600 ft (12,400 m).
Two XP-51B prototypes[nb 3]were adapted from P-51 airframes; these were a more thorough conversion than the Mustang X, with a tailor-made engine installation and a complete redesign of the radiator duct. The airframe itself was strengthened, with the fuselage and engine mount area receiving more formers because of the greater weight of the Packard V-1650-3, 1,690 lb (770 kg), compared with the Allison V-1710's 1,335 lb (606 kg). The engine cowling was completely redesigned to house the Packard Merlin, which, because of the intercooler radiator mounted on the supercharger casing, was 5 in (130 mm) taller and used an updraught induction system, rather than the downdraught carburetor of the Allison. The new engine drove a four-bladed 11 ft 2 in (3.40 m) diameter Hamilton Standard propeller that featured cuffs of hard molded rubber. To cater for the increased cooling requirements of the Merlin a new fuselage duct was designed. This housed a larger radiator, which incorporated a section for the supercharger coolant, and, forward of this and slightly lower, an oil cooler was housed in a secondary duct which drew air through the main opening and exhausted via a separate exit flap.
It was decided that the armament of the new P-51B (NA-102) would permanently omit the previously nose-mounted machine guns used on earlier P-51 versions, and only the four wing-mounted .50 in (12.7 mm) M2/AN Browning machine guns (with 350 rpg for the inboard guns and 280 rpg for the outboard) of the P-51A would be used for its gun armament. The bomb rack/external drop tank installation, adapted from the A-36 Apache attack version, would also be used; the racks were rated to be able to carry up to 500 lb (230 kg) of ordnance and were also capable of carrying drop tanks. The weapons were aimed using an N-3B optical gunsight fitted with an A-1 head assembly which allowed it to be used as a gun or bomb sight through varying the angle of the reflector glass. Pilots were also given the option of having ring and bead sights mounted on the top engine cowling formers. This option was discontinued with the later P-51Ds.
The first XP-51B flew on 30 November 1942. Although flight tests confirmed the potential of the new fighter, with the service ceiling being raised by 10,000 feet and the top speed improving by 50 mph at 30,000 ft (9,100 m), it was soon discovered that the radiator duct airflow was breaking up at high speeds, generating a rumble as the exit shutter was closed. Testing at the Ames Aeronautical laboratory led to a redesign of the radiator scoop culminating in a forward slanted upper lip. After sustained lobbying at the highest level, American production was started in early 1943 with the P-51B (NA-102) being manufactured at Inglewood, California, and the P-51C (NA-103) at a new plant in Dallas, Texas, which was in operation by summer 1943.[nb 4] The RAF named these models Mustang Mk III. In performance tests, the P-51B reached 441 mph (709.70 km/h) at 30,000 ft (9,100 m). [nb 5] In addition, the extended range made possible by the use of drop tanks enabled the Merlin-powered Mustang to be introduced as a bomber escort with a combat radius of 750 miles using two 75 gal tanks.
The range would be further increased with the introduction of an 85 gal (322 l) self-sealing fuel tank aft of the pilot's seat, starting with the P-51B-5-NA series. When this tank was full, the center of gravity of the Mustang was moved dangerously close to the aft limit. As a result, maneuvers were restricted until the tank was down to about 25 U.S. gal (95 l) and the external tanks had been dropped. Problems with high-speed "porpoising" of the P-51Bs and P-51Cs with the fuselage tanks would lead to the replacement of the fabric-covered elevators with metal-covered surfaces and a reduction of the tailplane incidence. With the fuselage and wing tanks, plus two 75 gal drop tanks, the combat radius was now 880 miles.
Despite these modifications, the P-51Bs and P-51Cs, and the newer P-51Ds and P-51Ks, experienced low-speed handling problems that could result in an involuntary "snap-roll" under certain conditions of air speed, angle of attack, gross weight, and center of gravity. Several crash reports tell of P-51Bs and P-51Cs crashing because horizontal stabilizers were torn off during maneuvering. As a result of these problems, a modification kit consisting of a dorsal fin was manufactured. One report stated:
"Unless a dorsal fin is installed on the P-51B, P-51C and P-51D airplanes, a snap roll may result when attempting a slow roll. The horizontal stabilizer will not withstand the effects of a snap roll. To prevent recurrence, the stabilizer should be reinforced in accordance with T.O. 01-60J-18 dated 8 April 1944 and a dorsal fin should be installed. Dorsal fin kits are being made available to overseas activities"
The dorsal fin kits became available in August 1944, and were fitted to P-51Bs and P-51Cs, and to P-51Ds and P-51Ks. Also incorporated was a change to the rudder trim tabs, which would help prevent the pilot over-controlling the aircraft and creating heavy loads on the tail unit.
P-51Bs and P-51Cs started to arrive in England in August and October 1943. The P-51B/P-51C versions were sent to 15 fighter groups that were part of the 8th and 9th Air Forces in England and the 12th and 15th in Italy (the southern part of Italy was under Allied control by late 1943). Other deployments included the China Burma India Theater (CBI).
Allied strategists quickly exploited the long-range fighter as a bomber escort. It was largely due to the P-51 that daylight bombing raids deep into German territory became possible without prohibitive bomber losses in late 1943.
A number of the P-51B and P-51C aircraft were fitted for photo reconnaissance and designated F-6C.
One of the few remaining complaints with the Merlin-powered aircraft was a poor rearward view. The canopy structure, which was the same as the Allison-engined Mustangs, was made up of flat, framed panels; the pilot gained access, or exited the cockpit by lowering the port side panel and raising the top panel to the right. The canopy could not be opened in flight and tall pilots especially, were hampered by limited headroom. In order to at least partially improve the view from the Mustang, the British had field-modified some Mustangs with clear, sliding canopies called Malcolm hoods (designed by Robert Malcolm). The new structure was a frameless plexiglas moulding[nb 6] which ballooned outwards at the top and sides, increasing the headroom and allowing increased visibility to the sides and rear. Because the new structure slid backwards on runners it could be slid open in flight. The aerial mast behind the canopy was replaced by a "whip" aerial which was mounted further aft and offset to the right. Most British Mk IIIs were equipped with Malcolm hoods. Several American service groups "acquired" the necessary conversion kits and some American P-51B/P-51Cs appeared with the new canopy, although the majority continued to use the original framed canopies.
A better solution to the problem was the "teardrop", or "bubble", canopy. Originally developed as part of the Miles M.20 project, these newer canopies were being adapted to most British designs, eventually appearing on Typhoons, Tempests and later-built Spitfires. In America new moulding techniques had been developed to form streamlined nose transparencies for bombers. North American designed a new streamlined plexiglass canopy for the P-51B which was later developed into the teardrop shaped bubble canopy. In late 1942 the tenth production P-51B-1-NA was removed from the assembly lines. From the windshield aft the fuselage was redesigned by cutting down the rear fuselage formers to the same height as those forward of the cockpit; the new shape faired in to the vertical tail unit. Wind tunnel tests of a wooden model confirmed that the aerodynamics were sound. A new simpler style of windscreen, with an angled bullet-resistant windscreen mounted on two flat side pieces improved the forward view while the new canopy resulted in exceptional all-round visibility.
A common misconception is that the cutting down of the rear fuselage to mount the bubble canopy reduced stability, requiring the addition of a dorsal fin to the forward base of the vertical tail. In fact, as described, stability problems affected the earlier P-51Bs and P-51Cs, as well as the subsequent P-51D/P-51K models; this was partly attributable to the 85 U.S. gal (322 l) fuselage fuel tank that had been installed during production of the P-51B-5-NA and caused the center of gravity to move back too far when filled. Other factors were the switch from the three-blade propeller of the Allison-powered series to the four-blade propeller, causing increased destabilization due to the four-bladed propeller's greater side area effect, and, on the P-51D and P-51K, the bubble canopy causing some turbulence ahead of the fin.[nb 7]
Among other modifications, armament was increased with the addition of two more M2 machine guns, bringing the total to six. The inner pair of machine guns had 400 rpg, and the others had 270 rpg, for a total of 1,880. In previous P-51s, the M2s were mounted at an extreme side angle to allow access to the feed chutes from the ammunition trays. This angled mounting had caused problems of congestion, jamming of the ammunition and spent casings and links, leading to frequent complaints of jamming during combat maneuvers. The new arrangement allowed the M2s to be mounted upright, remedying most of the jamming problems. The .50 in (12.7 mm) Browning machine guns, although not firing an explosive projectile, had excellent ballistics and proved adequate against the Focke-Wulf Fw 190 and Messerschmitt Bf 109 fighters, which were the main USAAF opponents at the time. The wing racks fitted to the P-51D/P-51K series were strengthened and were able to carry up to 1,000 lb (450 kg) of ordnance. Later models had under-wing rocket pylons added to carry up to ten rockets per plane.
Alterations to the undercarriage up-locks and inner-door retracting mechanisms meant that there was a change to the shape of the inner-wing leading edge, which was raked forward slightly, increasing the wing area and creating a distinctive "kink" in the leading edges of the wings.
The P-51D became the most widely produced variant of the Mustang. A Dallas-built version of the P-51D, designated the P-51K, was equipped with an Aeroproducts propeller in place of the Hamilton Standard propeller, as well as a larger, differently configured canopy and other minor alterations (the vent panel was different). The hollow-bladed Aeroproducts propeller was unreliable, with dangerous vibrations at full throttle due to manufacturing problems, and was eventually replaced by the Hamilton Standard. By the time of the Korean War, most F-51s were equipped with "uncuffed" Hamilton Standard propellers with wider, blunt-tipped blades. The photo reconnaissance versions of the P-51D and P-51K were designated F-6D and F-6K respectively. The RAF assigned the name Mustang Mk IV to the P-51D model and Mustang Mk IVA to P-51K models.
The P-51D/P-51K started arriving in Europe in mid-1944 and quickly became the primary USAAF fighter in the theater. It was produced in larger numbers than any other Mustang variant. Nevertheless, by the end of the war, roughly half of all operational Mustangs were still P-51B or P-51C models.
Concern over the USAAF's inability to escort B-29s all the way to mainland Japan resulted in the highly classified "Seahorse" project, an effort to "navalize" the aircraft. In late 1944, naval aviator (and later test pilot) Lieutenant Bob Elder flew carrier suitability trials aboard the carrier USS Shangri-La using P-51D 44-14017, which had been fitted with an arrestor hook. The project was canceled after U.S. Marines secured the Japanese island of Iwo Jima and its airfields, making it possible for standard P-51D models to accompany B-29s all the way to the Japanese home islands and back.
During 1945–48, P-51Ds were also built under licence in Australia by the Commonwealth Aircraft Corporation.
The USAAF required airframes built to their acceleration standard of 8.33 g (82 m/s²), a higher load factor than that used by the British standard of 5.33 g (52 m/s²) for their fighters. Reducing the load factor to 5.33 would allow weight to be removed, and both the USAAF and the RAF were interested in the potential performance boost.
A subtle change made in the lightweight Mustangs was the use of an improved NACA 66 series airfoil and a slightly thinner wing than that used by earlier Mustangs.
In 1943, North American submitted a proposal to redesign the P-51D as model NA-105, which was accepted by the USAAF. Modifications included changes to the cowling, a simplified undercarriage with smaller wheels and disc brakes, and a larger canopy. The designation XP-51F was assigned to prototypes powered with V-1650 engines (a small number of XP-51Fs were passed to the British as the Mustang V), and XP-51G to those with reverse lend/lease Merlin RM 14 SM engines.
A third lightweight prototype powered by an Allison V-1710-119 engine was added to the development program. This aircraft was designated XP-51J. Since the engine was insufficiently developed, the XP-51J was loaned to Allison for engine development. None of these experimental lightweights went into production.
The P-51H (NA-126) was the final production Mustang, embodying the experience gained in the development of the XP-51F and XP-51G aircraft. This aircraft, with minor differences as the NA-129, came too late to participate in World War II, but it brought the development of the Mustang to a peak as one of the fastest production piston-engine fighters to see service.
The P-51H used the new V-1650-9 engine, a version of the Merlin that included Simmons automatic supercharger boost control with water injection, allowing War Emergency Power as high as 2,218 hp (1,500 kW). Differences between the P-51D included lengthening the fuselage and increasing the height of the tailfin, which greatly reduced the tendency to yaw. The canopy resembled the P-51D style, over a somewhat raised pilot's position. Service access to the guns and ammunition was also improved. With the new airframe several hundred pounds lighter, the extra power and a more streamlined radiator, the P-51H was among the fastest propeller fighters ever, able to reach 487 mph (784 km/h or Mach 0.74) at 25,000 ft (7,600 m).
The P-51H was designed to complement the P-47N as the primary aircraft for the invasion of Japan, with 2,000 ordered to be manufactured at Inglewood. Production was just ramping up with 555 delivered when the war ended.
Additional orders, already on the books, were canceled. With the cutback in production, the variants of the P-51H with different versions of the Merlin engine were produced in either limited numbers or terminated. These included the P-51L, similar to the P-51H but utilizing the 2,270 hp (1,690 kW) V-1650-11 engine, which was never built; and its Dallas-built version, the P-51M, or NA-124, which utilized the V-1650-9A engine lacking water injection and therefore rated for lower maximum power, of which one was built out of the original 1629 ordered, serial number 45-11743.
Although some P-51Hs were issued to operational units, none saw combat in World War II, and in postwar service, most were issued to reserve units. One aircraft was provided to the RAF for testing and evaluation. Serial number 44-64192 was designated BuNo 09064 and used by the U.S. Navy to test transonic airfoil designs and then returned to the Air National Guard in 1952. The P-51H was not used for combat in the Korean War despite its improved handling characteristics, since the P-51D was available in much larger numbers and was a proven commodity.
Many of the aerodynamic advances of the P-51 (including the laminar flow wing) were carried over to North American's next generation of jet-powered fighters, the Navy FJ Fury and Air Force F-86 Sabre. The wings, empennage and canopy of the first straight-winged variant of the Fury (the FJ-1) and the unbuilt preliminary prototypes of the P-86/F-86 strongly resembled those of the Mustang before the aircraft were modified with swept-wing designs.
In early 1944, the first P-51A-1-NA, 43-6003. was fitted and tested with a lightweight retractable ski kit replacing the wheels. This conversion was made in response to a perceived requirement for aircraft that would operate away from prepared airstrips. The main oleo leg fairings were retained, but the main wheel doors and tail wheel doors were removed for the tests. When the undercarriage was retracted, the main gear skis were housed in the space in the lower engine compartment made available by the removal of the fuselage .50 in (12.7 mm) Brownings from the P-51As. The entire installation added 390 lb (180 kg) to the aircraft weight and required that the operating pressure of the hydraulic system had to be increased from 1,000 psi (6,897 kPa) to 1,200 psi (8,276 kPa). Flight tests showed that ground handling was good, and the Mustang could take off and land in a field length of 1,000 ft (300 m); the maximum speed was 18 mph (29 km/h) lower, although it was thought that fairings over the retracted skis would compensate.
On 15 November 1944, a navalized P-51D-5-NA, 44-14017, started flight tests from the deck of the carrier Shangri-La. This Mustang had been fitted with an arrestor hook, which was attached to a reinforced bulkhead behind the tail wheel opening; the hook was housed in a streamlined position under the rudder fairing and could be released from the cockpit. The tests showed that the Mustang could be flown off the carrier deck without the aid of a catapult, using a flap setting of 20° down and 5° of up elevator. Landings were found to be easy, and, by allowing the tail wheel to contact the deck before the main gear, the aircraft could be stopped in a minimum distance.
While North American were concentrating on improving the performance of the P-51 through the development of the lightweight Mustangs, in Britain, other avenues of development were being pursued. To this end, two Mustang Mk IIIs (P-51Bs and P-51Cs), FX858 and FX901, were fitted with different Merlin engine variants. The first of these, FX858, was fitted with a Merlin 100 by Rolls-Royce at Hucknall; this engine was similar to the RM 14 SM fitted to the XP-51G and was capable of generating 2,080 hp (1,550 kW) at 22,800 ft (7,000 m) using a boost pressure of +25 lbf/in2 (170 kPa; 80 inHg) in war emergency setting. With this engine, FX858 reached a maximum speed of 453 mph (729 km/h) at 18,000 ft (5,500 m), and this could be maintained to 25,000 ft (7,600 m). The climb rate was 4,160 ft/min (21.1 m/s) at 14,000 ft (4,300 m).
FX901 was fitted with a Merlin 113 (also used in the de Havilland Mosquito B. Mk 35). This engine was similar to the Merlin 100, but it was fitted with a supercharger rated for higher altitudes. FX901 was capable of 454 mph (730 km/h) at 30,000 ft (9,100 m) and 414 mph (666 km/h) at 40,000 ft (12,200 m).
At the Casablanca Conference, the Allies formulated the Combined Bomber Offensive (CBO) plan for "round-the-clock" bombing by the RAF at night and the USAAF by day. American pre-war bombardment doctrine held that large formations of heavy bombers flying at high altitudes would be able to defend themselves against enemy interceptors with minimal fighter escort, so that precision daylight bombing using the Norden bombsight would be effective.
Both the RAF and Luftwaffe had attempted daylight bombing and discontinued it, believing advancements in single-engine fighters made multi-engined bombers too vulnerable, contrary to Giulio Douhet's thesis. The RAF had worried about this in the mid-1930s and had decided to produce an all night-bomber force, but initially began bombing operations by day. The Germans used extensive daylight bombing during the Battle of Britain in preparation for a possible invasion. Due to the high casualty rates, the Luftwaffe soon switched to night bombing (see The Blitz). Bomber Command followed suit in its raids over Germany.
Initial USAAF efforts were inconclusive because of the limited scale. In June 1943, the Combined Chiefs of Staff issued the Pointblank Directive to destroy the Luftwaffe before the invasion of Europe, putting the CBO into full implementation. The 8th Air Force heavy-bomber force conducted a series of deep-penetration raids into Germany beyond the range of available escort fighters. German fighter reaction was fierce, and bomber losses were severe—20% in an October 14 attack on the German ball-bearing industry. This made it too costly to continue such long-range raids without adequate fighter escort.
The Lockheed P-38 Lightning had the range to escort the bombers, but was available in very limited numbers in the European theater due to its Allison engines proving difficult to maintain. With the extensive use of the P-38 in the Pacific Theater of Operations, where its twin engines were deemed vital to long-range "over-water" operations, nearly all European-based P-38 units converted to the P-51 in 1944. The Republic P-47 Thunderbolt was capable of meeting the Luftwaffe on more than even terms, but did not at the time have sufficient range. The Mustang changed all that. In general terms, the Mustang was at least as simple as other aircraft of its era. It used a single, well-understood, reliable engine and had internal space for a huge fuel load. With external fuel tanks, it could accompany the bombers all the way to Germany and back.
Enough P-51s became available to the 8th and 9th Air Forces in the winter of 1943-44, and, when the Pointblank offensive resumed in early 1944, matters changed dramatically. The P-51 proved perfect for the task of escorting bombers all the way to the deepest targets, thus complementing the more numerous P-47s until sufficient Mustangs became available. The Eighth Air Force immediately began to switch its fighter groups to the Mustang, first exchanging arriving P-47 groups for those of the 9th Air Force using P-51s, then gradually converting its Thunderbolt and Lightning groups until, by the end of the year, 14 of its 15 groups flew the Mustang.
Luftwaffe Experten were confident that they could out-manoeuvre the P-51 in a dogfight. Kurt Bühlingen, the third-highest scoring German fighter pilot of the Second World War on the Western Front, with 112 victories, later recalled that "We would out-turn the P-51 and the other American fighters, with the (Bf)109 or the (Fw)190. Their turn rate was about the same. The P-51 was faster than us but our munitions and cannon were better”. Robert S. Johnson, the second-highest scoring U.S. fighter pilot in European theatre flew the P-47 against German fighters. Johnson pointed out “Generally speaking, I’d say the best (Focke-Wulf) 190s and the P-51 were very close in performance; the difference was probably in the pilot in these combats.” 
Usually Luftwaffe pilots attempted to avoid U.S. fighters by massing in huge numbers well in front of the bombers, attacking in a single pass, then breaking off the attack, allowing escorting fighters little time to react. The need to inflict heavy casualties on the American bombers was now more pressing than ever. To do this, the German fighters needed to carry very heavy armament. The weight of this armament decreased performance to the point where their aircraft were sitting ducks if caught by the P-51s, likely to appear in large numbers anywhere over Germany. The Luftwaffe answer was the Gefechtsverband (battle formation). It consisted of a Sturmgruppe of heavily armed and armoured Fw 190s escorted by two Begleitgruppen of light fighters, often Bf 109Gs, whose task was to keep the Mustangs away from the Fw 190s attacking the bombers. This scheme was excellent in theory but difficult to apply in practice. The massive German formation took a long time to assemble and was difficult to manoeuvre. It was often intercepted by the escorting P-51s and broken before reaching the bombers. But when the Sturmgruppe worked, the effects were devastating. With their engines and cockpits heavily armoured, the Fw 190s attacked from astern and gun camera films show that these attacks were often pressed to within 100 yds.
While not always successful in avoiding contact with the escort (as the tremendous loss of German pilots in the spring of 1944 indicates), the threat of mass attacks, and later the "company front" (eight abreast) assaults by armored Sturmgruppe Fw 190s, brought an urgency to attacking the Luftwaffe wherever it could be found. The P-51, particularly with the advent of the K-14 gunsight and the development of "Clobber Colleges" for the in-theater training of fighter pilots in fall 1944, was a decisive element in Allied countermeasures against the Jagdverbände.
Beginning in late February 1944, 8th Air Force fighter units began systematic strafing attacks on German airfields that picked up in frequency and intensity throughout the spring, with the objective of gaining air supremacy over the Normandy battlefield. In general, these were conducted by units returning from escort missions, but beginning in March, many groups also were assigned airfield attacks instead of bomber support. On 15 April, VIII FC began Operation Jackpot, attacks on specific Luftwaffe fighter airfields, and on 21 May, these attacks were expanded to include railways, locomotives, and rolling stock used by the Germans to transport materiel and troops, in missions dubbed "Chattanooga". The P-51 also excelled at this mission, although losses were much higher on strafing missions than in air-to-air combat, partially because, like other fighters using liquid-cooled engines, the Mustang's coolant system could be punctured by small arms hits, even from a single bullet.
The numerical superiority of the USAAF fighters, superb flying characteristics of the P-51 and pilot proficiency helped cripple the Luftwaffe's fighter force. As a result, the fighter threat to US, and later British bombers, was greatly diminished by July 1944. Reichmarshal Hermann Göring, commander of the German Luftwaffe during the war, was quoted as saying, "When I saw Mustangs over Berlin, I knew the jig was up."
P-51s also distinguished themselves against advanced enemy rockets and aircraft. A P-51B/P-51C with 150 octane fuel was fast enough to pursue the V-1s launched toward London. The Messerschmitt Me 163 rocket interceptors and Messerschmitt Me 262 jet fighters were considerably faster than the P-51, but were not as maneuverable as the Mustang, and as all aircraft are, were vulnerable on take-off and landing. Chuck Yeager, flying a P-51D, was one of the first American pilots to shoot down a Me 262 when he surprised it during its landing approach. On 7 October 1944, Lt. Urban Drew of the 365th Fighter Group went him one better. During a fighter sweep, he surprised and shot down two Me 262s taking off. On the same day, Hubert Zemke, now flying Mustangs, shot down what he thought was a Bf 109, only to have his gun camera film reveal it to be an Me 262. On 1 November 1944, the Mustang pilots once again demonstrated that the threat could be contained with numbers. While flying as escorts for B-17s, the 20th Fighter Group was attacked by a lone Me 262, which destroyed a solitary P-51. The Me 262 then attempted to attack the bombers, only to be cut off by a mixed formation of P-51s and P-47s. The fighter groups competed for the kill. Eventually, a P-47 pilot of the 56th, and Mustang pilots Lts. Gerbe and Groce of the 352nd Fighter Groups, shared the kill.
By 8 May 1945, the 8th, 9th and 15th Air Forces' P-51 groups [nb 8] claimed some 4,950 aircraft shot down (about half of all USAAF claims in the European theater), the most claimed by any Allied fighter in air-to-air combat , and 4,131 destroyed on the ground. Losses were about 2,520 aircraft.
One of these groups, the 8th Air Force's 4th Fighter Group, was the overall top-scoring fighter group in Europe, with 1,016 enemy aircraft claimed destroyed. This included 550 claimed in aerial combat and 466 on the ground.
In aerial combat, the top-scoring P-51 units (both of which exclusively flew Mustangs) were the 357th Fighter Group of the 8th Air Force with 595 air-to-air combat victories, and the Ninth Air Force's 354th Fighter Group with 701, which made it the top scoring outfit in aerial combat of all fighter groups of any type. Martin Bowman reports that in the European Theater of Operations, Mustangs flew 213,873 sorties and lost 2,520 aircraft to all causes. The top Mustang ace was the USAAF's George Preddy, whose tally stood at 27.5, 24 scored with the P-51 when he was shot down and killed by friendly fire on Christmas Day 1944 during the Battle of the Bulge.  The P-51s were deployed in the Far East later in 1944, operating in close-support and escort missions as well as for tactical photo reconnaissance.
In the aftermath of World War II, the USAAF consolidated much of its wartime combat force and selected the P-51 as a "standard" piston-engine fighter, while other types, such as the P-38 and P-47, were withdrawn or given substantially reduced roles. However, as more advanced jet fighters (P-80 and P-84) were being introduced, the P-51 was relegated to secondary status.
In 1947, the newly-formed USAF Strategic Air Command employed Mustangs alongside F-6 Mustangs and F-82 Twin Mustangs, due to their range capabilities. In 1948, the designation P-51 (P for pursuit) was changed to F-51 (F for fighter), and the existing F designator for photographic reconnaissance aircraft was dropped because of a new designation scheme throughout the USAF. Aircraft still in service in the USAF or Air National Guard (ANG) when the system was changed included: F-51B, F-51D, F-51K, RF-51D (formerly F-6D), RF-51K (formerly F-6K), and TRF-51D (two-seat trainer conversions of F-6Ds). They remained in service from 1946 through 1951. By 1950, although Mustangs continued in service with the USAF after the war, the majority of the USAF's Mustangs had been surplussed or transferred to the Air Force Reserve (AFRES) and the Air National Guard (ANG).
During the Korean War, F-51s, though obsolete as fighters, were used as close ground-support aircraft and reconnaissance aircraft until the end of the war in 1953. Because of its lighter structure and less availability of spare parts, the newer, faster F-51H was not used in Korea. With the aircraft being used for ground attack, their performance was less of a concern than their ability to carry a load.
At the start of the Korean War, the Mustang once again proved its usefulness. With the availability of F-51Ds in service and in storage, a substantial number were shipped via aircraft carriers to the combat zone for use initially by both the Republic of Korea Air Force (ROKAF) and USAF. Rather than employing them as interceptors or "pure" fighters, the F-51 was given the task of ground attack, fitted with rockets and bombs. After the initial invasion from North Korea, USAF units were forced to fly from bases in Japan, and F-51Ds could hit targets in Korea that short-ranged F-80 jet fighters could not. A major concern over the vulnerability of the cooling system was realized in heavy losses due to ground fire. Mustangs continued flying with USAF and ROKAF fighter-bomber units on close support and interdiction missions in Korea until they were largely replaced by Republic F-84 and Grumman Panther jet fighter-bombers in 1953. No. 77 Squadron Royal Australian Air Force (RAAF) operated Australian-built Mustangs as part of British Commonwealth Forces Korea, replacing them with Gloster Meteor F8s in 1951. No. 2 Squadron South African Air Force (SAAF) operated US-built Mustangs as part of the US 18th Fighter Bomber Wing, suffering heavy losses by 1953, when it converted to the F-86 Sabre.
F-51s flew in the Air Force Reserve and Air National Guard throughout the 1950s. The last American USAF Mustang was F-51D-30-NA AF Serial No. 44-74936, which was finally withdrawn from service with the West Virginia Air National Guard in 1957. This aircraft is now on display at the National Museum of the United States Air Force at Wright-Patterson AFB in Dayton, Ohio. It is, however, painted as P-51D-15-NA Serial No. 44-15174.
The final withdrawal of the Mustang from USAF dumped hundreds of P-51s out onto the civilian market. The rights to the Mustang design were purchased from North American by the Cavalier Aircraft Corporation, which attempted to market the surplus Mustang aircraft both in the U.S. and overseas. In 1967 and again in 1972, the USAF procured batches of remanufactured Mustangs from Cavalier, most of them destined for air forces in South America and Asia that were participating in the Military Assistance Program (MAP). These aircraft were remanufactured from existing original F-51D airframes but were fitted with new V-1650-7 engines, a new radio fit, tall F-51H-type vertical tails, and a stronger wing that could carry six 0.50 in (13 mm) machine guns and a total of eight underwing hardpoints. Two 1,000 lb (454 kg) bombs and six 5 in (127 mm) rockets could be carried. They all had an original F-51D-type canopy, but carried a second seat for an observer behind the pilot. One additional Mustang was a two-seat dual-control TF-51D (67-14866) with an enlarged canopy and only four wing guns. Although these remanufactured Mustangs were intended for sale to South American and Asian nations through the MAP, they were delivered to the USAF with full USAF markings. They were, however, allocated new serial numbers (67-14862/14866, 67-22579/22582 and 72-1526/1541).
The last U.S. military use of the F-51 was in 1968, when the U. S. Army employed a vintage F-51D (44-72990) as a chase aircraft for the Lockheed YAH-56 Cheyenne armed helicopter project. This aircraft was so successful that the Army ordered two F-51Ds from Cavalier in 1968 for use at Fort Rucker as chase planes. They were assigned the serials 68-15795 and 68-15796. These F-51s had wingtip fuel tanks and were unarmed. Following the end of the Cheyenne program, these two chase aircraft were used for other projects. One of them (68-15795) was fitted with a 106 mm recoilless rifle for evaluation of the weapon's value in attacking fortified ground targets. Cavalier Mustang 68-15796 survives at the Air Force Armament Museum, Eglin AFB, Florida, displayed indoors in World War II markings.
The F-51 was adopted by many foreign air forces and continued to be an effective fighter into the mid-1980s with smaller air arms. The last Mustang ever downed in battle occurred during Operation Power Pack in the Dominican Republic in 1965, with the last aircraft finally being retired by the Dominican Air Force (FAD) in 1984.
After World War II, the P-51 Mustang served in the air arms of more than 55 nations. During wartime, a Mustang cost about 51,000 dollars , while many hundreds were sold postwar for the nominal price of one dollar to the American countries that signed the Inter-American Treaty of Reciprocal Assistance, ratified in Rio de Janeiro in 1947. Following is a list of some of the countries that used the P-51 Mustang.
The Philippines acquired 103 P-51D Mustangs after World War II. These became the backbone of the postwar Philippine Army Air Corps and Philippine Air Force and were used extensively during the Huk campaign, fighting against Communist insurgents. Mustangs were also the first aircraft of the Philippine air demonstration squadron, which was formed in 1953 and given the name "The Blue Diamonds" the following year. The Mustangs were replaced by 50 F-86 Sabres in the late 1950s, but some were still in service for COIN roles up to the early 1970s.
The Somalian Air Force had brought 8 P-51D models in service.
It also served with the ROKAF Black Eagles aerobatic team, until retired 1954.
Many P-51s were sold as surplus after the war, often for as little as $1,500. Some were sold to former wartime fliers or other aficionados for personal use, while others were modified for air racing.
One of the most significant Mustangs involved in air racing was a surplus P-51C-10-NT (44-10947) purchased by Paul Mantz, a film stunt pilot. The aircraft was modified by creating a "wet wing", sealing the wing to create a giant fuel tank in each wing, which eliminated the need for fuel stops or drag-inducing drop tanks. This Mustang, called Blaze of Noon, came in first in the 1946 and 1947 Bendix Air Races, second in the 1948 Bendix, and third in the 1949 Bendix. He also set a U.S. coast-to-coast record in 1947. The Mantz Mustang was sold to Charles F. Blair Jr (future husband of Maureen O'Hara) and re-named Excaliber III. Blair used it to set a New York-to-London (c. 3,460 mi/5,568 km) record in 1951. Later that same year, he flew from Norway to Fairbanks, Alaska, via the North Pole (c. 3,130 mi/5,037 km), proving that navigation via sun sights was possible over the magnetic north pole region. For this feat, he was awarded the Harmon Trophy, and the Air Force was forced to change its thoughts on a possible Soviet air strike from the north. This Mustang now resides in the National Air and Space Museum at Steven F. Udvar-Hazy Center.
The most prominent firm to convert Mustangs to civilian use was Trans-Florida Aviation, later renamed Cavalier Aircraft Corporation, which produced the Cavalier Mustang. Modifications included a taller tailfin and wingtip tanks. A number of conversions included a Cavalier Mustang specialty: a "tight" second seat added in the space formerly occupied by the military radio and fuselage fuel tank.
In 1958, 78 surviving RCAF Mustangs were retired from service's inventory and were ferried from their varied storage locations to Canastota, New York. These aircraft make up a large percentage of the aircraft presently flying worldwide.
In the late 1960s and early 1970s, when the United States Department of Defense wished to supply aircraft to South American countries and later Indonesia for close air support and counter insurgency, it turned to Cavalier to return some of their civilian conversions back to updated military specifications.
In the twenty-first century a P-51 can command a price of more than $1 million, even for only partially restored aircraft. Some privately owned P-51s are still flying, often associated with organizations such as the Commemorative Air Force (formerly the Confederate Air Force).
In early 1942, the USAAF ordered a lot of 500 aircraft modified as dive bombers that were designated A-36A. North American assigned the aircraft the model number NA-97. This model became the first USAAF Mustang to see combat. One aircraft was passed to the British, who gave it the name Mustang Mk I (Dive Bomber).
Following the A-36A order, the USAAF ordered 310 model NA-99 fighters that were designated P-51A by the USAAF and Mustang Mk II by the RAF. A number of this lot of aircraft were equipped with K-24 cameras and designated F-6B. All these models of the Mustang were equipped with Allison V-1710 engines except the prototype XP-51B.
Beginning with the model NA-102 Mustang, the Packard V-1650 replaced the Allison. In the summer of 1943, Mustang production was begun at a new plant in Dallas, Texas, as well as at the existing facility in Inglewood, California. The model NA-102 was produced as the P-51B in Inglewood, while the NA-103 as the P-51C was produced at Dallas. The RAF named these models Mustang Mk III. Again, a number of the P-51B and P-51C aircraft were fitted for photo Reconnaissance and designated F-6C.
The prototypes of the bubble canopy change were designated model NA-106 by North American and P-51D by the USAAF. The production version, while retaining the P-51D designation, was assigned model number NA-109 by North American. The P-51D became the most widely produced variant of the Mustang. A variation of the P-51D equipped with an Aeroproducts propeller in place of the Hamilton Standard propeller was designated the P-51K. The photo versions of the P-51D and P-51K were designated F-6D and F-6K respectively. The RAF assigned the name Mustang Mk IV to the P-51D model and Mustang Mk IVA to P-51K models.
As the USAAF specifications required airframe design to a higher load factor than that used by the British for their fighters, consideration was given to re-designing the Mustang to the lower British requirements in order to reduce the weight of the aircraft and thus improve performance. In 1943, North American submitted a proposal to do the re-design as model NA-105, which was accepted by the USAAF. The designation XP-51F was assigned for prototypes powered with V-1650 engines and XP-51G to those with reverse lend/lease Merlin 145M engines. Modifications included changes to the cowling, a simplified undercarriage with smaller wheels and disk brakes, and a larger canopy. A third prototype was added to the development that was powered by an Allison V-1710 engine. This aircraft was designated XP-51J. As the engine was insufficiently developed, the XP-51J was loaned to Allison for engine development. A small number of XP-51Fs were passed to the British as the Mustang Mk V.
The final production Mustang, the P-51H, embodied the experience gained in the development of the lightweight XP-51F and XP-51G aircraft. This aircraft, model NA-126, and, with minor differences, NA-129, came too late to participate in World War II, but it brought the development of the Mustang to a peak and was one of the fastest production piston-engine fighters to see service. The P-51H used the Merlin V-1659-9 engine, equipped with Simmons automatic boost control and water injection, allowing War Emergency Power as high as 2,218 hp (1,654 kW). Some of the weight savings inherited from the XP-51F and XP-51G were invested in lengthening the fuselage and increasing the height of the tailfin, greatly reducing the tendency to yaw, and in restoring the fuselage fuel tank. The canopy was changed back to more nearly resemble the P-51D style, over a somewhat raised pilot's position. Service access to the guns and ammunition was improved. The P-51H was designed to complement the P-47N as the primary aircraft for the invasion of Japan, and 2,000 were ordered to be built at the Inglewood plant. With the solution to the problem of yaw control, the P-51H was now considered a suitable candidate for testing as an aircraft carrier-based fighter; but with the end of the war, the testing was cut short, and production was halted after 555 aircraft were built. Although some P-51Hs were issued to operational units, none saw combat. One aircraft was given to the RAF for testing and evaluation. Serial number 44-64192 was re-serialed as BuNo 09064 and used by the U.S. Navy to test transonic airfoil designs, then returned to the Air National Guard in 1952. The P-51H was not used for combat in the Korean War despite its improved handling characteristics, due to the lack of experience with durability of the lighter airframe under combat conditions as well as limited numbers in the USAF inventory.
With the cutback in production, the variants of the P-51H with different versions of the Merlin engine were produced in either limited numbers or terminated. These included the P-51L, similar to the P-51H but utilizing the 2,270 hp (1,690 kW) V-1650-11 engine, which was never built, and its Dallas-built version, the P-51M, or NA-124, which utilized the V-1650-9A engine lacking water injection and therefore rated for lower maximum power, of which one was built out of the original 1629 ordered, AAF Serial Number 45-11743.
Total number built: 16,766 (most numerous American fighter aircraft)
The P-51 has been the subject of numerous scale flying replicas; aside from ever-popular R/C-controlled aircraft, several kitplane manufacturers offer ½, ⅔, and ¾-scale replicas capable of comfortably seating one (or even two) and offering high performance combined with more forgiving flight characteristics. Such aircraft include the Titan T-51 Mustang, W.A.R. P-51 Mustang, Linn Mini Mustang, Jurca Mustang, Thunder Mustang, and Loehle 5151 Mustang.
Among the 287 current airframes and the 154 "flying" Mustangs are the following:
Data from The Great Book of Fighters