There were only two XB-70s built. In 1965, while stationed at Offutt AFB, Nebraska, I was with the Civil Air Patrol. We took the CAP Cadets on a trip to Edwards AFB, California and had the privilege of seeing BOTH XB-70s parked, nose to nose, in a hanger. Security restrictions at the time did not allow us to take pictures, but it is a memory I will never forget. I found this tie tack in an antique shop awhile back.
Here also are a few links about the aircraft.
http://en.wikipedia.org/wiki/North_American_XB-70_Valkyrie
http://www.youtube.com/watch?v=Lfh-lV_jhzk&feature=related
http://www.youtube.com/watch?v=_vPvr29wJ80&feature=related