Over twenty variants of the North American P Mustang fighter were produced from , when it first flew, to after World War II , some of which were employed also in the Korean War and in several other conflicts.
The prototype of the Mustang, designated NAX, was rolled out by North American Aviation on 9 September , albeit without an engine, and was first flown the following 26 October. Unlike later models, Allison-powered Mustangs were characterized by the carburetor air intake placed on the dorsal surface of the nose, immediately behind the propeller.
Contractual arrangements were also made for two aircraft from the first order to be delivered to the USAAC for evaluation; these two airframes, and respectively, were designated XP 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, ft 4, m.
Two XPs serials and set aside for testing arrived at Wright Field on 24 August and 16 December respectively. After the late-June reorganization of the USAAC into the United States Army Air Forces , roughly six months before the attack on Pearl Harbor changed the outlook for the United States regarding involvement in global hostilities against the Axis overnight , priority had to be given to building as many existing fighters — Ps, Ps, and Ps — as possible while simultaneously training pilots and other personnel, which meant evaluation of the XP did not begin immediately.
However, this did not mean it was neglected, or testing and evaluation mishandled. It was quickly evident the Mustang's performance, although exceptional up to 15, ft 4, m the supercharger's critical altitude rating , was markedly reduced at higher altitudes. This was because the single-speed single-stage supercharger fitted to the V had been designed to produce maximum power at low altitude; above that, power dropped off rapidly.
