The XV-15 Tilt Rotor Research Aircraft was a joint NASA/U.S. Army/Bell Helicopter program to demonstrate the benefits of the tilt rotor concept. (The piston-powered XV-3 had demonstrated feasibility but was too underpowered to allow a valid evaluation.) First flown on 3 May 1977, the flight envelope was eventually expanded to 300 knots in airplane mode. After the initial aerodynamic and aeroelastic research objectives had been accomplished, the two XV-15s were used for mission demonstrations, to include nap-of-the-earth flight, shipboard landings, and air-to-air combat maneuvering. Because the XV-15 was easy to fly, a guest-pilot program was initiated to provide hands-on tilt rotor experience to aviator as well as non-aviator decision makers. The very successful evaluation and demonstration effort resulted in the V-22 Osprey program.
Further reading: NASA XV-15 monograph, The History of the XV-15 Tilt Rotor Research Aircraft: From Concept to Flight, 2000.