National Park Service aircraft, autogiro, Kellett KD-1 at Boulder City Airport, May 5, 1941, Pilot Mos Brair, The Kellett KD-1 was a 1930s American autogyro built by the Kellett Autogiro Company. It had the distinction of being the first practical rotary-wing aircraft used by the United States Army and inaugurated the first scheduled air-mail service using a rotary-wing aircraft