In 1937, USAAC lieutenants Gordon P. Saville and Benjamin S. Kelsey devised a pair of proposals for interceptor aircraft, the first such designation in the US. One proposal was for a single-engined fighter, the other for a twin-engine. Both were required to reach an altitude of in six minutes as a defense against bomber attack. Kelsey said later that he used the "interceptor" designation to sidestep a hard USAAC policy restricting fighters to 500 lbs of armament. He wished for at least of armament so that American fighters could dominate their battles against all opponents, fighters included. The two aircraft resulting from these proposals were the single-engine Bell P-39 Airacobra and the twin-engine Lockheed P-38 Lightning. Both aircraft were successful during World War II in standard fighter roles, not specifically assigned to point defense against bombers.