As the first international X-plane program, the Defense Advanced Research Projects Agency (DARPA) was the US Government's top-level management, with the US Navy serving as the military agent through Naval Air Systems Command (NAVAIR).

[1] The X-31 design was essentially an all-new airframe design, although it borrowed heavily on design elements and sometimes actual parts of previous production, prototype, and conceptual aircraft designs, including the British Aerospace Experimental Aircraft Programme (choice of wing type with canards, plus underfuselage intake), the German TKF-90 (wing planform concepts and underfuselage intake), F/A-18 Hornet (forebody, including cockpit, ejection seat, and canopy; electrical generators), F-16 Fighting Falcon (landing gear, fuel pump, rudder pedals, nosewheel tires, and emergency power unit), F-16XL (leading-edge flap drives), V-22 Osprey (control surface actuators), Cessna Citation (main landing gear's wheels and brakes), F-20 Tigershark (hydrazine emergency air-start system, later replaced) and B-1 Lancer (spindles from its control vanes used for the canards).

To reduce the cost of tooling for a production run of only two aircraft, Rockwell developed the "fly-away tooling" concept (perhaps the most successful spinoff of the program), whereby 15 fuselage frames were manufactured via CNC, tied together with a holding fixture, and attached to the factory floor with survey equipment.

The X-31 featured a cranked-delta wing (similar to the Saab 35 Draken and the F-16XL prototype), and fixed strakes along the aft fuselage, as well as a pair of movable computer-controlled canards to increase stability and maneuverability.

On April 29, 1993, the second X-31 successfully executed a rapid minimum-radius, 180° turn using a post-stall maneuver, flying well outside the range of angle of attack normal for conventional aircraft.

[7] VECTOR is a joint venture that includes the US Navy, Germany's defense procurement agency BWB, Boeing's Phantom Works, and DASA; it was initially expected to involve Sweden, which pulled out due to fiscal constraints.