The initial design (the A-12) actually was designed and paid for by the CIA for reconaissance. However, the subsequent difficulties of the B-58 jet and ASG-18 missile gave Lockheed the idea to adapt the A-12 design for continental defense.
And so, it was adapted into the YF-12 design, and was going to have a number of these (93 actually) interceptors stationed stateside, to guard against Russian high-speed, low-level bombers. Their speed would allow them to get to the point of incursion quickly. It was to be armed with the XAIM-47A. Turns out Robert S#!^head McNamara preferred the F-106, and so that was the end of the YF-12 variant.
So Lockheed came back and proposed it as a stategic reconaissance bomber, designated the RS-12. Once again, McNamara saw as a threat to the ailing B-70 project. He declared there was no need for a Mach 3+ weapons delivery platform, but the RS designation was retained "for political reasons." It was at this point that Johnson flubbed his classified speech and referred to the SR, rather than the RS program. As a result, the designation was hastily and officially changed. And while my reference source ("Lockheed SR-71 Blackbird", by Paul F. Crickmore, 1986, Osprey: London) does not explain how it got from "12" to "71", it might have just been for disinformation reasons.
BTW, you bring to mind another Pentagon scandal: the retirement of the Blackbird.
I can only presume that, despite the hoopla, we have something even better. I recall hearing vague things about something called Aurora in the media around that time. My brother-in-law was a blue-suiter and later a Lockheed rep who spent his career working on the high-end Lockheed products (that is how I got this great reference book autographed by many of the pilots, RSO's, techs and even the Det 4 commander). My brother-in-law can "neither confirm nor deny" the existence of Aurora. I only in recent years found out (upon declassification) that my Dad, while at GE, designed the 1st successful reentry heat shield (after others had failed) for the original US spy satellite (Discoverer). So I can only guess that the US had an continues to have all sorts of reconaissance platforms to choose from.
But I agree, I sure miss the SR-71, which I had several of opportunities to observe up close, on the ground and in flight, over the years. What a feat of engineering it remains to this day.
By the way, I highly recommend the reference book I cited in this post, if you have interest in the Blackbird.
Thanks for the very informed reply, X. I'll check out Crickmore. I've seen the 'bird on the Intrepid, but never in flight. IMO, just the flat-out coolest work of engineering that ever made it off the drafting table. Too bad the original plan wasn't pursued--might have shortened the Cold War.
Aurora, huh? Interesting. Sounds like you come from a very distinguished defense family.