[427TJ]

I think the difference was that the XB-70 was only able to sustain high Mach numbers for relatively short periods whereas the SR-71 could do so for longer periods. Also, by the time the XB-70 first flew, ICBMs had taken over the prominent role in the U.S. Strategic Integrated Operations Plan (SIOP) and thus the XB-70 was basically obsolete as it rolled out the factory doors. Just as with the battleships of the U.S. Navy, the idea of the XB-70 was overtaken by technology, at least as far as the Cold War was concerned. Battleships did have one last hurrah during the 1991 Gulf War as they lobbed "Volkswagens" (WWII-era 2,000 pound cannon shells) into Kuwait but otherwise they were long out of date. Perhaps the XB-70, had it been in use in 1991, could also have pummeled the Iraqi Republican Guard, free from the threat of Iraqi SAMs and interceptors.