The P-64 designation falls out of chronological order and belongs to a North American product the USAAC never intended to use. The company Model NA-50A, ordered by Siam on 30 December 1939, was little more than a single-seat pursuit ship patterned after the Harvard trainer and developed from the NA-50 used by Peru. The six examples of this strictly export craft were built at Inglewood and painted in Siamese markings, and were en route to Siam when the US Army confiscated them, removed the armament, and assigned them to training duties at Luke Field, Arizona. A widely-published report that the Siam-bound aircraft were caught at Pearl Harbor during the 7 December 1941 Japanese attack is inaccurate: the NA-50As were apparently embargoed in October 1940 and a camouflaged example in USAAF markings was noted at Luke as early as 16 September 1941.

Never really a fighter in USAAF service, the six P-64s were essentially base 'hacks' and possibly never received American serial numbers. A privately-owned survivor in civil registry as N840 was airworthy in the US as recently as 1975.