If you are even implying that the XB-70 was cancelled because of the fear of the MIG-25 -- Stop.
The XB-70 was cancelled because the jet -- believe it or not -- could not live up to its conceptual expectations. In other words, it was too technically demanding of a project and the USAF needed the funds elsewhere.
The XB-70 was designed around the idea of 'compression lift'...
https://en.wikipedia.org/wiki/Compression_lift
http://www.aerospaceweb.org/design/waverider/design.shtml
http://xb70.interceptor.com/
Essentially, the jet would be 'surfing' on its own supersonic shock waves. The XB-70 was approaching that goal when program technical difficulties and mishaps forced the project's cancellation. In many ways, the XB-70's goals were more technically challenging than the F-35's.
The MIG-25's speed was achieved thru sheer propulsive brute strength. No subtlety or finesse about it.
The SR-71's speed was achieved thru minimal aerodynamic drag via extraordinary shaping.
The XB-70 combined both.
If finances allowed, the XB-70 would -- not could or might -- end up as the world's fastest and highest altitude cruising bomber. Not even the MIG-25 could chase.