By the late 1940's, bell Aircraft began designing the convertiplane concept. It was a fixed-wing airplane that could take off like a helicopter. It had two large prop-rotors that mounted on each wing tip . The rotors were tilted horizontally for level flight. In the 1950's, the U.S. Air Force and Army announced a competition for a convertiplane, in which Bell submitted their model 200 and was awarded a contract in 1951. The two protptypes were designated the XV-3 and XH-33.The XV-3 made its' first hover flight in 1955, but crashed 2 months later during testing. The problem was found to be a flutter problem with the original three-blade rotors, so a second XV-3 was built with a 2-blaed rotor system, as well as some other detail changes. The second XV-3 made it first flight in 1958, and became the first aircraft in history to fully tilt its rotors from vertical to horizontal while in flight. The second XV-3 made over 250 flights before it was damaged in a wind tunned test in 1965.