McDonnell's earlier work with helicopters had led to exploring vertical takeoff and landing (VTOL) aircraft, starting with the Air Force Experimental XV-1. Called "Convertiplane," the XV-1 used stub wings and a pusher propeller for forward thrust to augment vertical lift from the main rotor. The XV-1 made its first in-flight conversion from helicopter to aircraft in April 1954 and was the first rotorcraft to fly more than 200 mph.