The Lightweight Fighter (LWF) program was a United States Air Force technology evaluation program initiated in the late 1960s by a group of officers and defense analysts known as the "Fighter Mafia". It was spurred by then-Major John Boyd's 'energy-maneuverability' (E-M) theory, which indicated that excessive weight would have severely debilitating consequences on the maneuverability of an aircraft. Boyd's design called for a light-weight fighter with a high thrust-to-weight ratio, high maneuverability, and a gross weight of less than 20,000 lb (9,100 kg), half that of its counterpart, the McDonnell Douglas F-15 Eagle. It resulted in the development of the General Dynamics YF-16 and Northrop YF-17. Late in the program, in 1974, with the promise of European sales, the Air Force changed the program name to Air Combat Fighter (ACF), and committed to purchasing 650 models of the YF-16, adopted as the F-16 Fighting Falcon. The U.S. Navy adopted a modified version of the YF-17 as the McDonnell Douglas F/A-18 Hornet.

Project Forecast, a 1963 Air Force study, attempted to identify future weapons trends and "certain high-priority areas for research and development (R&D), recommendations based on the greatest potential payoff for the future." The report strongly suggested that future air combat would be carried out primarily by long-range missile fire. Future "fighters" would be designed primarily for long range, high speed, and equipped with extremely large radar systems in order to detect and engage opposing fighters at beyond visual range (BVR). This made them much more like interceptors than classic fighter designs, and led to increasingly heavier and more technologically sophisticated designs – and thus costlier.

The US Navy had long ago come to similar conclusions, and had been designing a series of aircraft dedicated to this role. Notable among these was the well named Douglas F6D Missileer, a long endurance but slow and unmaneuverable design equipped with very powerful missiles and radar for fleet defense. The US Air Force had similar designs, but these had been dedicated to the interceptor role, where the large size of its targets allowed reasonable radar performance. As radar equipment improved, in particular the introduction of Doppler radar systems, fighters gained similar range performance against smaller targets. By the early 1960s, even before the release of Forecast, both the Air Force and Navy expected to use the General Dynamics F-111 Aardvark (then still in development as the TFX) and McDonnell Douglas F-4 Phantom II for their long- and medium-range needs. The perception of a declining need for close-in "dogfighting" capabilities resulted in the original decision to not install internal cannons in the Phantom.

However, real-world experience in the Vietnam War revealed some shortcomings in American fighter capabilities, as early generation Soviet jet fighters proved to be more of a challenge than expected for U.S. designs. Although U.S. pilots had achieved favorable kill-to-loss ratios, combat had revealed that air-to-air missiles (AAM) of this era were significantly less reliable than anticipated. Furthermore, the rules of engagement in Vietnam precluded long-range missile attacks in most instances, as visual identification was normally required. Under these conditions, combat invariably closed to short ranges where maneuverability and short-range air-to-air weapons became critical, even for dedicated interceptors like the Convair F-102 Delta Dagger.

Based on his experiences in the Korean War and as a fighter tactics instructor, in the early 1960s Colonel John Boyd and mathematician Thomas Christie developed the Energy-Maneuverability (E-M) theory of the value of aircraft specific energy maintenance as an advantage in fighter combat. Maneuverability was the means of getting “inside” an adversary’s decision-making cycle, a process Boyd called the "OODA" loop (from "Observation-Orientation-Decision-Action"). This approach emphasized an aircraft design capable of “fast transients” – quick changes in speed, altitude, and direction. A fighter that is superior in its ability to gain or lose energy while out-turning an opponent can initiate and control any engagement opportunity; a fast transient capability allows the pilot to stay inside a hard-turning opponent when on the offensive or to force an overshoot of an opponent when on the defensive.

These parameters called for a small, lightweight aircraft – which would minimize drag and increase the thrust-to-weight ratio – but a larger, higher-lift wing to minimize wing loading – which tends to reduce top speed while increasing payload, and can lower range (which can be compensated for by increased fuel in the larger wing).

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%.

Boyd used his E-M theory to argue that the F-X design was far too heavy, and this led to new requirements for a smaller F-X in the 40,000-pound (18,100 kg) class. However, the appearance of the Mach-3-capable MiG-25 in July 1967 led to serious concerns within the Defense Department that the Soviets had developed a super fighter capable of outrunning anything the US fielded, while its huge wing would make it highly maneuverable. In response, the ADF effort was essentially ignored in favour of an all-out effort to develop a superior F-X, which would emerge as the F-15 Eagle.