 Two aircraft were built to prove the tiltrotor design and explore the operational flight envelope for military and civil applications.In 1981, using experience gained from the XV-3 and XV-15, Bell and Boeing Helicopters began developing the V-22 Osprey, a twin-turboshaft military tiltrotor aircraft for the U.S. Air Force and the U.S. Marine Corps.Bell teamed with Boeing in developing a commercial tiltrotor, but Boeing went out in 1998 and Agusta came in for the Bell/Agusta BA609.