I was thinking that too. Just last night I read an Aviation History article on the XB-70 (Edit, here’s the article online: LeMay’s Dream Bomber ). It appears that cost was the big factor. The XB-70 was hugely expensive and SecDef McNamara was sold on ICBMs as a replacement for manned bombers. Survivability was one excuse, but not the determining factor the the big bomber’s demise.
North American and Boeing were also asked to give the bomber low level capability in the initial design request. I’ve never read about how well the North American final design worked at low level though.
Back to the original question, one of these would look awesome with the SR-71 paint scheme.
I’ve always been enamored by the Valkyrie. With the canards, and long fuselage it kind of reminds me of an angry viper. In a lot of ways this aircraft is even more impressive than the Habu.