bypass the inflexible Army Air Corps requirement for pursuit aircraft to carry no more than of armament including ammunition, and to bypass the USAAC restriction of single-seat aircraft to one engine. Kelsey was looking for a minimum of of armament. Kelsey and Saville aimed to get a more capable fighter, better at dog fighting and at high-altitude combat. Specifications called for a maximum airspeed of at least at altitude, and a climb to within six minutes, the toughest set of specifications USAAC had ever presented. The unbuilt Vultee XP1015 was designed to the same requirement, but was not advanced enough to merit further investigation. A similar proposal for a single-engined fighter was issued at the same time, Circular Proposal X-609, in response to which the Bell P-39 Airacobra was designed. Both proposals required liquid-cooled Allison V-1710 engines with turbosuperchargers and gave extra points for tricycle landing gear. Lockheed formed a secretive engineering team to implement the project apart from the main factory; this approach later became known as Skunk Works. The Lockheed design team, under the direction of Hall Hibbard and Clarence "Kelly" Johnson, considered a range of twin-engined configurations, including both engines in a central fuselage with push–pull propellers. The eventual configuration was rare in terms of contemporary production fighter aircraft design, with the Dutch Fokker G.I heavy fighter, and the later Northrop P-61 Black Widow night fighter and Swedish SAAB 21 having a similar planform. The Lockheed team chose twin booms to accommodate the tail assembly, engines, and turbosuperchargers, with a central nacelle for the pilot and armament. The XP-38 gondola mockup was designed to mount two .50-caliber (12.7 mm) M2 Browning machine guns with 200 rounds per gun (rpg), two .30-caliber (7.62 mm) Brownings with 500 rpg, and a United States Army Ordnance Department prototype T1 23 mm (.90 in) autocannon with a rotary magazine as a substitute for the nonexistent 25 mm Hotchkiss aircraft autocannon specified by Kelsey and Saville. In the prototype YP-38s, an Army Ordnance Department T9 37 mm (1.46 in) autocannon (later designated as the M4 in production) with 15 rounds replaced the 23 mm T1. The 15 rounds were in three five-round clips, an unsatisfactory arrangement according to Kelsey, and the T9/M4 did not perform reliably in flight. Further armament experiments from March to June 1941 resulted in the P-38E combat configuration of four M2 Browning machine guns, and one Hispano 20