The North American Aviation P-51 Mustang is an American long-range, single-seat fighter and fighter-bomber used during World War II and the Korean War, among other conflicts. The Mustang was designed in 1940 by a team headed by James H. Kindelberger of North American Aviation (NAA) in response to a requirement of the British Purchasing Commission. The commission approached NAA to build Curtiss P-40 fighters under license for the Royal Air Force (RAF). Rather than build an old design from another company, NAA proposed the design and production of a more modern fighter. The prototype NA-73X airframe was completed on 9 September 1940, 102 days after contract signing, achieving its first flight on 26 October.
The Mustang was designed to use the Allison V-1710 engine without an export-sensitive turbosupercharger or a multi-stage supercharger, resulting in limited high-altitude performance. The aircraft was first flown operationally by the RAF as a tactical-reconnaissance aircraft and fighter-bomber (Mustang Mk I). In mid 1942, a development project known as the Rolls-Royce Mustang X, replaced the Allison engine with a Rolls-Royce Merlin 65 two-stage inter-cooled supercharged engine. During testing at Rolls-Royce's airfield at Hucknall in England, it was clear the engine dramatically improved the aircraft's performance at altitudes above 15,000 ft (4,600 m) without sacrificing range. Following receipt of the test results and after further flights by USAAF pilots, the results were so positive that North American began work on converting several aircraft developing into the P-51B/C (Mustang Mk III) model, which became the first long-range fighter to be able to compete with the Luftwaffe's fighters. The definitive version, the P-51D, was powered by the Packard V-1650-7, a license-built version of the two-speed, two-stage-supercharged Merlin 66, and was armed with six .50 caliber (12.7 mm) AN/M2 Browning machine guns.
