The first XV-3. Photo date given as 14 October 1955 by the Dutch source but Wikipedia suggests the aircraft did not fly between August 1955 and late March 1956 due to a hard landing. 54-147 was damaged beyond repair on 25 October 1956 when it shaked so badly that the pilot blacked out. Photo from: Nationaal Archief (Netherlands)

On display in the Research & Development Gallery at the National Museum of the U.S. Air Force. This was the world's first VSTOL tilt-rotor aircraft, powered by a single P&W R-985 Wasp. It was the result of a joint initiative in 1951 between the US Army and USAF. Two were built, and the first hover tests began in 1955. Take off to horizontal flight and back was achieved on 18 Dec 58. It never went into production, but paved the way later for the CV-22 Osprey. The first XV-3 was written off, and this is the only surviving example.

The second XV-3 hovering in front of building N-211 at Moffett Field. The XV-3 was the first tiltrotor design to achieve full tilting. With one 450-hp Wasp Junior engine it was underpowered, however, and unable to hover out of ground effect. Photo from: NASA

Bell Aircraft began studying the tilt rotor concept in the 1940s. The XV-3 was the first hardware to come out of the design work and led to the V-22 Osprey (via the Bell XV-15 tilt rotor). Two XV-3s were built between 1952 -55, with the first (54-147) crashing in 1956 during testing. The cause was in-flight instability and the remaining aircraft continued extensive flight and wind tunnel testing. This aircraft was retired in 1966 after being damaged during the last wind tunnel test. It was transferred to the National Museum of the US Air Force in 2007 after a two year restoration by Bell.