The need for new air superiority fighters led the USAF to initiate two concept development studies in 1965: the Fighter Experimental (F-X) project originally envisioned a 60,000-pound (27,200 kg) class twin-engine design with a variable-geometry wing. This would be a very expensive aircraft, too expensive to consider equipping the entire Air Force with it as its standard aircraft. For this reason a less-expensive Advanced Day Fighter (ADF) was also considered, a lightweight design in the 25,000 lb (11,300 kg) class which would out-perform the MiG-21 by 25%.