In 1940, the Royal Thai Air Force ordered six aircraft similar to the NA-50 that were designated NA-68. The changes in the NA-68 included a modified landing gear, new outer wings, heavier armament, and redesigned tail surfaces similar to those adopted on later production trainers. North American test pilot Lewis Waite flew the first NA-68 on 1 September 1940.

In 1940, the NA-68s (along with a parallel order for NA-69 two seaters) ordered by the Royal Thai Air Force were en route to Thailand when their export clearance was cancelled and were returned to the United States where they were assigned the designation P-64, disarmed, and used for advanced fighter training.