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.

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.

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.

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.

[8] 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.

General Dynamics received $49,000 and Northrop $100,000 study contracts to design concepts that embodied Boyd's E-M theory – a small, low-drag, low-weight, pure fighter with no bomb racks; their work would lead to the YF-16 and YF-17, respectively.

The Request for Proposals (RFP) was issued 6 January 1972, and called for a 20,000 lb (9,100 kg) class fighter with a good turn rate, acceleration and range, and optimized for combat at speeds of Mach 0.6–1.6 and altitudes of 30,000–40,000 ft (9,150–12,200 m).

Since one of the goals of the program was to validate emerging technologies, Secretary of the Air Force Robert Seamans chose to select the General Dynamics and Northrop entries.

First, four North Atlantic Treaty Organization (NATO) members – Belgium, Denmark, the Netherlands, and Norway – were looking to replace their F-104G fighter-bomber variants of the Lockheed F-104 Starfighter interceptor; furthermore, they were seeking an aircraft that their own aerospace industries could manufacture under license, as they had the F-104G.

Secondly, while the USAF was not particularly interested in a complementary air superiority fighter competing with F-X, it did need to begin replacing its F-4 and Republic F-105 Thunderchief fighter-bombers.

These requirements meshed relatively well, but the timing of the procurement was driven by the timeframe needs of the four allies, who had formed a "Multinational Fighter Program Group" (MFPG) and were pressing for a U.S. decision by December 1974.

[15][16][17][18] ACF also raised the stakes for GD and Northrop because it brought in further competitors intent on securing the lucrative order that was touted at the time as "the arms deal of the century".

On 11 September 1974, the U.S. Air Force confirmed firm plans to place an order for of the winning ACF design sufficient to equip five tactical fighter wings.

[21] The chief reasons given by the Secretary for the decision were the YF-16's lower operating costs; greater range; and maneuver performance that was "significantly better" than that of the YF-17, especially at near-supersonic and supersonic speeds.