First swing-wing aircraft

The first variable-geometry aircraft (a plane that can change the sweep angle of its wings in flight) was the Bell X-5. In take-off configuration, the X-5 had wings swept back 20 degrees, but it could increase this to 40 degrees and even 60 degrees to improve flight characteristics at higher transonic speeds. The X-5 first flew on 20 June 1951 at Edwards Air Force Base, in California, USA.

As aircraft get into what are known as transonic speeds (anything from around Mach 0.72 to Mach 1.0), straight wings start to cause aerodynamic problems. At these higher speeds, wings that are steeply swept back perform much better. However, these steeply swept-back wings make low-speed flight (particularly around take-off and landing) unstable and difficult. Variable-geometry wings allow the pilot to adjust the plane's layout to whatever is most appropriate for each stage of flight.

There are two earlier designs that are sometimes mentioned in the context of variable-geometry aircraft.

The first is the Westland-Hill Pterodactyl series, a group of unique tailless aircraft built in the United Kingdom during the 1920s and 1930s. One of these prototypes reportedly featured the ability to slightly vary the sweep of its wings in order to adjust longitudinal trim during flight. As the variable sweep was serving an entirely different aerodynamic purpose to the other designs, it's not considered a comparable development. Also, the technical details of this design, including how much variability there was in the wings, have been lost.

The second is the Messerschmitt P.1101, an experimental German aircraft from 1944, which featured variable sweep wings. This aircraft never flew, however, and the sweep of the wings was only adjustable on the ground before each flight. It was, however, a significant influence on the design of the Bell X-5.