It was initially ordered by Sweden, but by the time the aircraft were ready for delivery in 1941, the United States would not allow them to be exported, designating them as P-66s and retaining them for defensive and training purposes.

Vultee's chief engineer Richard W. Palmer, made an all out effort to secure substantial military orders on the eve of World War II.

The aircraft featured a metal-covered, semi-monocoque fuselage and fully retractable landing gear, and was powered by a Pratt & Whitney R-1830 air-cooled radial engine.

During construction of the first prototype, a decision was made to lengthen the propeller shaft and install a tight cowling to provide a pointed nose to reduce drag.

[4] Although pilots were impressed by the P-66's handling, the type was considered less than robust and a tendency to ground-loop led to 15 aircraft being destroyed in landing accidents.

After concluding their trials however, the British then relinquished the aircraft to China where 104 Vanguards (including USAAC examples) were shipped under the Lend-Lease program.

However, many P-66s were destroyed on the ground during Japanese attacks while several were shot down in error when they were mistaken for the Nakajima firm's Ki-43 "Oscar" and Ki-44 "Tojo".