TU-160 Blackjack

Production might have been cancelled in 1961, but the fact that they were still flying them until 1969 could've been alarming to the Soviets. Obviously they assumed the testing would lead to an operational aircraft with high speed capability. I think it's safe to say development of the MiG-25 could've started as far back as the F-108 Rapier, even if it never flew.

Yeah, that makes no sense. The XB-70 was a research program from 1961 onwards. A good deal of the research was related to the US SST program, which we ended up cancelling anyway. There already was a legitimate military Mach 3 aircraft under development for operational use, the Blackbird series. Why would the Soviets want to design an aircraft to shoot down a research aircraft? If that's the case, where was their Mach 6 interceptor designed to shoot down the X-15? There are a number of inaccuracies still being propagated through the media and the defense establishment, but that doesn't mean they're right. The U-2 proved vulnerable to SAMs. The US developed an aircraft to be immune to existing SAM systems. Logic would dictate the Soviets would have to develop something to counter that, and it was the MiG-25P.