Bell X-5 (signed)The X 5 was the world's first airplane to vary the sweepback of its wings in flight. It was built to prove the theory that by increasing the sweepback of an airplane's wings after takeoff, a higher maximum speed could be obtained, while still retaining low takeoff and landing speed and higher rate of climb with the wings swept forward. The X 5 was based upon the design of a Messerschmitt P. 1101 airplane discovered in Germany at the end of World War