I really enjoyed doing the research on the YF-12A. I thought it was a spin off of the SR-71. I was wrong. The original spy plane was the A-12. It was a single seat airplane that looked like the SR-71 we are all familiar with. Next came the YF-12A. The three of them were built in a corner of the Lockheed Skunk Works with the A-12s. A second cockpit was added to the airframe for the radar operator. The SR-71 came next. It kept the second cockpit. The 2 major visual difference are the YF-12A has a round radome.(the chine does not extend clear out to the tip, as on the SR-71) and the YF-12A has a shorter tail cone than the SR-71.
The YF-12 was meant to be a fast interceptor armed with 4 (some references say 3) Hughes AIM-47A nuclear missiles. It would launch, intercept the Soviet bomber force at mach 3 and blow them away with nukes! The test program was ended in 1969. However, the two surviving aircraft were brought out of storage and loaned to NASA. One crashed (#936) and one (#935) was flying into 1979. It (#935) was then retired to the Air Force Museum on November 7, 1979.
This aircraft #935 is built and painted as it appeared in early 1963 with no camera pods. The two tone finishes did not last long. After 1963 all three aircraft were finished in the overall black or very dark blue; (depending on the reference source).