The North American Aviation P-51 Mustang was an American long-range single-seat World War II fighter aircraft. It gained the reputation of being one of the best, if not the best, fighters of the war. Mustang pilots claimed 4,950 aircraft shot down, the most of any Allied fighter in the conflict. It was conceived, designed and built by North American Aviation, under the direction of lead engineer Edgar Schmued, (NAA) in response to a specification issued directly to NAA by the British Purchasing Commission; the prototype NA-73X airframe was rolled out, although still minus engine, 102 days after the contract was signed. The Mustang was first designed to use a low-altitude rated Allison V-1710 engine, and was mainly used initially by the Royal Air Force (RAF) as a tactical-reconnaissance aircraft and fighter-bomber. The definitive version, the P-51D, was powered by the Packard V-1650-7, a licence-built version of the Rolls-Royce Merlin 60 series two-stage two-speed supercharged engine, and armed with six .50 caliber (12.7 mm) M2 Browning machine guns. From late 1943, P-51Bs (supplemented by P-51Ds from mid-1944) were used by the USAAF's Eighth Air Force to escort bombers in raids over Germany, while the RAF's 2 TAF and the USAAF's Ninth Air Force used the Merlin-powered Mustangs as fighter-bombers, roles in which the Mustang helped ensure Allied air superiority in 1944. The P-51 was also in service with Allied air forces in the North African, Mediterranean and Italian theatres, and saw limited service against the Japanese in the Pacific War. At the start of Korean War the Mustang was the United Nations' main fighter, a role that was soon shouldered by jet fighters, including the F-86. The Mustang became a specialised ground-attack fighter-bomber. In spite of being superseded by jet fighters, the Mustang remained in service with some air forces until the early 1980s. After World War II and the Korean War, many Mustangs were converted for civilian use, especially air racing.