Engineering Section. In this role he was the only person responsible for Air Corps fighter development, and was kept busy with inquiries and proposals from aircraft manufacturers. Kelsey continued to research blind landing techniques and develop instrument flying practices and hardware. On October 1, 1934 he was promoted to first lieutenant. Allison V-1710 Kelsey stayed current with efforts by Allison Engine Company to create a liquid-cooled engine suitable for fighters—such engines were seen as offering the possibility of greater speed and higher altitude than air-cooled engines. In late 1936 to early 1937, Kelsey flight-tested the 12-cylinder Allison V-1710-C6 as installed in the experimental Consolidated XA-11A, reaching an altitude of 26,400 ft without turbo-supercharging. The V-1710 passed difficult Air Corps endurance and reliability tests, and demonstrated a smooth, predictable power curve. Kelsey would subsequently base important fighter specifications on engines of this series. As Project Officer for Fighters, Kelsey tested a great number of aircraft models, possibly flying more new types of US fighters than any other pilot. He was first to fly the Bell twin-Allison XFM-1 Airacuda prototype on September 1, 1937. P-39 Airacobra Kelsey became frustrated by inflexible Air Corps restrictions on pursuit (fighter) aircraft which limited the weight of all guns and ammunition to 500 lbs. He wished for at least of armament so that American fighters could dominate their battles. With Lieutenant Gordon P. Saville, Kelsey formulated two new high-altitude fighter specifications in February 1937; one for a single-engine fighter and one for a twin-engine fighter. In Circular Proposal X-608 and X-609, the two men employed the word "interceptor" which had not yet been applied to American military aircraft specifications. In this way, they bypassed the Air Corps limitations. Despite the new term, the proposed aircraft were not fundamentally different in role than traditional pursuit aircraft—just heavier and more powerful. Kelsey specified Allison V-1710 engines with General Electric turbo-superchargers and tricycle landing gear in both proposals, aiming for performance, long range, the ability to climb to in six minutes, and very heavy armament including a cannon. Bell Aircraft won the single-engine X-609 contract with their design of the P-39 Airacobra. After the prototype flew, Kelsey was called to England, and handed oversight of the project off to colleagues who, in keeping with advice from the National Advisory Committee for Aeronautics (NACA), ordered the removal of turbo-superchargers from the aircraft. The P-39 was from that point forward unable