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Development
In the spring of 1940 the British Purchasing Commission approached North American Aviation to produce Curtis-designed P-40s for them. While North American had never built a fighter before (they had produced the T-6 / Harvard trainer and B-25 Mitchell medium bomber), NAA’s president said they could produce an entirely new fighter in less time than it would take to set up a production line for the P-40.
The new aircraft was to be built to British specifications, using features deemed most desirable in early 1940 fighter design, including a stressed skin metal construction and self-sealing fuel tanks. They also included an armament of four .303 calibre (7.62mm) machine guns, the Allison V-1710 inline engine (the same engine used by the P-40) and a flying prototype within 120 days.
The first prototype, designated NA-73X, was produced and rolled out within 117 (working) days, albeit without an engine. It utilized a new laminar flow wing. This design had nearly equal camber (curvature) on the top and bottom of the wing, and pushed the thickest part of the wing farther aft, which greatly reduced drag at higher airspeeds. Also, its radiator exhaust was routed such that it could be used as a form of jet thrust (the Meredith effect), similar to the Supermarine Spitfire. The first flight occurred on 26 October, 1940, with the British ordering 300 and designating them “Mustang Mk. I”.
Mustang Mk Is were first flown by British pilots on 1 May, 1941 as tactical reconnaissance platforms, armed with only four 12.7mm (.50 cal) machine guns. Even though the aircraft exhibited good response and excellent performance below 15,000ft (4570 m), its performance dropped considerably above that. Because of this, the British assigned these first Mustangs to low-level reconnaissance and ground attack duties. By the end of 1942, most production Mustangs were either Mk.I a’s, sporting four 20mm cannons, or Mk.II models
The USAAF took notice and received two models for evaluation purposes, designating them as XP-51. An initial order commitment was passed on until General Hap Arnold intervened, reserving at least 55 Mustangs bound for Britain. These aircraft were re-designated as F-6A and used for photo-reconnaissance. These were the first Mustangs operated for the USAAF, seeing deployment in North Africa in early 1943.
Full production of the base model P-51 for the USAAF began in March 1942. Armed with four 20 mm cannon, an excellent airframe but an under-performing Allison engine, the aircraft were utilized well in low-level operations. The A-36 Apache represented a dedicated ground-attack / dive-bomber version on the P-51 and was ordered in April 1942. Apaches were armed with six 12.7mm (.50 cal) machine guns, with two to each wing and two in the upper portion of the nose. This design first flew in September 1942, and put into action in June 1943.
Despite its usefulness as a low-level intruder, the Mustang had not lived up to the high-altitude specifications originally laid out by the British. Ronald Harker, a British test pilot, was impressed with the Mustang when he flew it in April 1942. He suggested that it would be a natural fit with the Rolls Royce Merlin 60-series engine. The Merlin engine was common to the Supermarine Spitfire and Hawker Hurricane and was well suited to high altitude flight. The United States, working along the same lines (using the Packard-built version of the Merlin) produced the P-51B, which first flew in November 1942. It included these changes:
Results of these tests yielded tremendous performance gains. This new Mustang was clocked at 441 miles per hour (710 km/h) at 30,000ft (9100 m) – 100 miles per hour (161 km/h) faster than the previous Mustang. These new Mustangs were designated Mustang Mk.III (RAF use) and P-51Bs and P-51Cs. The major difference between the B and C model was the place of origin - P-51Bs were produced at the Inglewood, California plant, whereas P-51Cs were produced at Dallas, Texas. The USAAF, desperately needing a long-range bomber escort, contracted for 2,200 P-51Bs and 1,750 P-51Cs.
After production of the B/C model began, three more changes appeared:
Even though the P-51B/C Mustangs out-performed the previous versions, they still had an issue in that the pilot had poor visibility to the rear due to the framed cockpit “razorback” design. With feedback from operational pilots, the P-51D model was developed, giving the Mustang its classic appearance with its teardrop canopy design, giving the pilot nearly a 360 degree field of view. Armament was also increased to six 12.7mm (.50cal) machine guns with a simplified ammunition feed system, reducing jams. The 85-gallon fuselage tank was also included; however, directional problems worsened in the first D models. To correct this, a dorsal fin was added.
Based on the improved Packard Merlin V-1659-9 engine, the fastest Mustang (P-51H) was introduced to production in June 1944. The fuselage was lengthened by two feet and the rudder and fin were increased in height. Other changes were made to the air intake, canopy, fuselage fuel tank (reduced to 50 gallons/190 litres) and the radiator ducts. This model was originally intended for the invasion of Japan. Some were issued to operational units in the Pacific before V-J day, but none entered combat. 555 of the H models were built before the program was cancelled in 1946. In Korea, the heavier and stronger D model was preferred.
Production of all 15,469 Mustangs was completed in 1946. With the change from USAAF to USAF in 1948, all remaining P-51 Mustangs were re-designated F-51s (“P” for Pursuit, “F” for Fighter).
World War II Operational Service
P-51Bs started appearing in Europe in December 1943, flown by the 354th Fighter Group. The Luftwaffe, unfamiliar with bomber escorts like the Mustang, employed Bf-110 Zerstorer, with little success. By early 1944, P-51s (especially the new D models) were being pressed into service as bomber escort fighters, fighter-bombers, and reconnaissance platforms.
While success of the D-models in Europe unfolded, P-51Ds eventually found their way to the Pacific and South East theatres by late 1944. Their primary role became escorting the new, long-range B-29 Superfortresses on their way to Japan and back.
Quick Facts
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