Bell XV-3
The Bell XV-3 was a tilt-rotor research aircraft built by Bell Helicopter for a joint U.S. Army and Air Force program exploring the convertiplane idea: an aircraft that takes off and lands like a helicopter but cruises like an airplane. Two were built. Between 1955 and 1966 the XV-3 became the first tilt-rotor to convert fully between vertical and horizontal flight, and although rotor instability and limited performance kept it a testbed, its data shaped the tilt-rotor designs that followed, through the XV-15 to the V-22 Osprey.