Here is some Youtube footage (courtesy of SonicBomb.com) of the XB-70 Valkyrie, North American Aviation's Mach 3 nuclear bomber prototype. Designed in the early '60s, this awesome flying machine was obsolete before it left the ground.
Despite it's incredible performance, the Valkyrie was designed as a high altitude bombing platform, specifically to strike the Soviet Union. Development of SAMs (surface to air missiles) made this approach to delivering nuclear weapons invalid - the aircraft would have been shot down before it reached the target.
Two XB-70s were completed. They were used as test and research platforms. One was lost in a mid-air collision during a photo-op. The other survived, and is currently enshrined at the USAF Museum in Dayton.
Some folks think this thing is wierd looking. I think it is one of the most beautiful large scale aircraft ever built.