If Aviation Leak - I mean, Aviation Week - is to be believed, the United States developed a two-stage-to-orbit vehicle. The first stage involved a modified B-70 airframe. The second stage was one of two small orbiters. Reportedly, it was operational in the 1990s.
This certainly would explain why the SR-71 was retired in 1990. This sounds an awful lot like the Aurora program that was also widely discussed in that timeframe. Funding probably was hidden inside the X-30 project (the NASP/COPPER CANYON), and some may have been diverted from the A-12 as well.