 The prototype of the Mustang, designated NA-73X, was rolled out by North American Aviation on 9 September 1940, albeit without an engine, and was first flown the following 26 October.The Mustang was originally designed to use a low-altitude rated Allison V-1710 engine.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.