The purpose of the X-31 was to develop methods of achieving a high degree of maneuverablity at high speed. In high speed flight, the aircraft becomes more sluggish and hard to control due to the hundreds of pounds of pressure from the air hitting the control surfaces. This aircraft uses thrust vectoring in the tailpipe of the engine, enabling post-stall maneuvers as well as a higher degree of maneuverability at high speed. The X-31 has already been developed as an experimental aircraft, but the US is thinking about using it for a fighter.

Specs on the X-31