Corps placed an order for four P-30 fighters and four A-11 attack aircraft. These production variants differed from the prototypes in having stronger fuselages, simplified undercarriages and more powerful engines. Operational history The first P-30 was delivered in January 1934. Testing showed that the gunner's cockpit was uncomfortable and cold at the high altitudes where the P-30 was intended to fight, while the rearward facing gunners were liable to black out when the aircraft was maneuvered. Despite these concerns, on 6 December 1934, the U.S. Air Corps placed an order for a further 50 P-30As, with more powerful V-1570-61 engines driving a three-bladed variable-pitch propeller and with oxygen supplies for the crew. Three of the four P-30s were delivered to the 94th Pursuit Squadron at Selfridge Field in 1934. The first P-30A, by this time redesignated PB-2A (Pursuit, Biplace), made its maiden flight on 17 December 1935, with deliveries to service units starting on 28 April 1936. The last of the 50 PB-2As were completed by August that year. While intended as a high altitude fighter, the PB-2 flew relatively few high altitude flights, partly because of the discomfort for the crew. One exception took place in March 1937, when a PB-2A was flown to before being forced to return to lower altitudes when the aircraft's controls froze. On 17 October 1936, a PB-2A flown by Lt. John M. Sterling won the Mitchell Trophy air race with a speed of . Since the PB-2A was one of the few aircraft at the time to have retractable landing gear, they were frequently damaged in "wheels-up" landings when the pilots forgot to extend the landing gear. One PB-2A was modified to a single-seat configuration as the PB-2A Special, to compete in a 1936 Air Corps competition for a new fighter to replace the Boeing P-26 Peashooter. It was larger and heavier than the other competitors and was much more expensive. It crashed during testing, with the Seversky P-35 being ordered into production. One A-11 was converted to the XA-11A testbed with the new 1,000 hp (746 kW) Allison XV-1710-7 engine. While the PB-2 was sturdy, the two-seat fighter concept was obsolete by the time the aircraft entered service, and by 1939, all had been replaced in front-line service by Seversky P-35 and Curtiss P-36 Hawk aircraft. The survivors remained in use as training aircraft until after the start of World War II,