Forty-three percent of the empty weight of the Rockwell/MBB X-31 is made up of over 600 items from other aircraft. In an effort to reduce costs and development time, off-the-shelf components were used wherever possible in the design and construction of the X-31.
McDD F/A-18 Hornet: Windscreen, canopy, electrical generators, airframe-mounted accessory gearbox, leading edge actuators, cockpit instrumentation, cockpit controls
Lockheed F-16: Main undercarriage, rudder pedals, nosewheel/tire, emergency power unit, fuel pump
Lockheed F-16XL: leading edge flap drives
Lockheed C-130 HTTB: Flight control computers
Bell/Boeing V-22: Rudder acutators, trailing edge control modules
Cessna Citation III: Mainwheels and brakes
Vought A-7: Main tires
Rockwell T-2: Zero-g fuel accumulator
Northrop F-20: Emergency air-start system
Rockwell B-1B: Canard pivot and spindle
Source: World Air Power Journal, Volume 24 (Spring 1996). "Rockwell/MBB X-31" by Robert F. Dorr.