The USAAC was satisfied with the low-altitude performance of the Airacobra, and directed that the twelve YP-39s be completed without turbosuperchargers. It was decided to embody the changes made in the XP-39B in the service test machines. effectively transforming them into YP-39B aircraft, but I don't know if this designation change ever became official.
The YP-39A was originally to have been delivered with the high-altitude V-1710-31 engine, but the change in USAAC philosophy caused this plan to be abandoned and this aircraft was delivered as a standard YP-39.
The first YP-39 (40-027) was flown on September 13, 1940 with the 1090 hp V-1710-37 (E5) engine driving a Curtiss Electric propeller. It differed externally from the XP-39B primarily in having a wider-chord vertical tail. The first few YP-39s were initially flown without armament, but subsequent machines were fitted with a 37 mm cannon with 15 rounds, a pair of 0.5-inch machine guns with 200 rounds per gun, and two 0.30-inch machine guns with 500 rounds per gun. All of these guns were mounted in the nose. Some armor protection was provided for the pilot. Empty and normal loaded weights rose to 5042 pounds and 7000 pounds, respectively. In comparison, the XP-39 prototype had a normal loaded weight of only 5550 pounds. Consequently, the performance of the YP-39 dropped to a maximum speed of 368 mph at 15,000 feet. An altitude of 20,000 feet could be attained in 7.3 minutes. Service ceiling was 33,300 feet.
The thirteen YP-39s (40-027/40-039) were delivered between September 6 and December 16, 1940. They were used primarily for evaluation and testing. Some of them were returned to the Bell factory for use in the development of improved Airacobra versions. These service-test machines apparently never reached Army aquadrons. Most of the YP-39s were lost in flight testing accidents.