So I was reading a rather forgettable book titled "Warplanes of the Future", by MBI; and there was this passage:
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Nine airframe companies and three engine manufacturers responded to the challenge when the USAF Aeronautical Systems Division (ASD) re-entered the game and issued its request for information (RFI) for an ATF in 1981.
At this early stage of the program, the USAF had not decided whether the new aircraft would emphasize air-to-air or air-to-ground missions and invited industry to share ideas for the new fighter.
The companies submitted a wide range of configurations in their responses. Lockheed favored a derivitive of the YF-12A, the forerunner of the two-seat SR-71 Blackbird. The YF-12A, designed for air-to-ground missions, carried several kinetic-energy penetrator weapons in a central weapon bay which would be released at supersonic speeds at high altitudes and guided to the target by a laser. The approach, which was worked through in early 1982, built up technical data gathered from a series of air-to-air missile launches from the YF-12A conducted in the mid-to-late 1960s. The YF-12A had fired seven Hughes AIM-47 missiles at altitudes of up to 80,000 feet (24,250m) at speeds of over Mach 3. The shots, at aerial targets at ranges of over 30 miles (48km), were highly successful. This high-altitude, high-speed approach was also one of Lockheed's candidates for the F-X program which became the F-15.
. . . .
Including stealth set an unusual security precedent. The security level of the original RFPs at this stage of the program precluded any details on stealth, a topic that was highly classified in the early 1980s. Companies that could claim low-observable (LO) technologies would be considered in a design but they could not reference any actual experience or techniques in their proposals. Stealth technologies were considered "black" and as such did not exist to anyone not cleared on them. The last minute change in the RFPs placed the program in both the "black" and "white" worlds.
The next phase in the program was the demonstration/validation (dem/val) phase in which companies would have to prove their technologies and refine their designs. Lockheed, however, took a radical departure from its high-speed, high-altitude design. "Clearly, ATF was going to be superstealth and not a cousin of the YF-12 or SR-71," explained Bart Osborne, Lockheed's chief engineer during this phase of the ATF program. "I stopped the YF-12 derivative effort and we started working on an F-117 derivative for ATF...."
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I'm really interested in the YF-12 derivatives they were looking at now...
