Interesting, but lets put to rest the myth that it couldn't be shot down. This was true thru most of it's life. By the end of it, its usefulnes as a spy ppane was limited to short trips over the USSR, lest it would have to deal with Migs 25 and 31 armed with Vympels R-33. A few close encounters that forced the planes to divert to the Mediterranean or Sweden (instead of just accelerating) pretty much sealed the deal.
On the allied side, the F-14 Tomcat with AIM-54 Phoenix missiles was capable of such feat. Incidentaly, the Phoenix was dwveloped for th YF-12, the prototype tha gave origin to the SR-71.