On 8 June 1966, XB-70 AV/2 (Air Vehicle 2) took part in a publicity flight
involving five aircraft powered by General Electric engines, over the
Mojave Desert.  The aircraft were flying in a V formation; from left to
right, a Northrop T-38A Talon, a McDonnell Douglas F-4B Phantom II, the
North American XB-70 Valkyrie, a Lockheed F-104N Starfighter, and a
Northrop F-5A Freedom Fighter.  Two more aircraft, a Learjet and an F-104D,
carried photographers.

At 9:26 a.m., just after the end of the photo session, with no warning or
explanation, the F-104N's tail hit the XB-70's right wingtip.  The F-104
flipped over and passed above the XB-70, slicing off parts of both of its
vertical fins on the way, then rammed into its port wing several times.
The XB-70 then flipped over and began spinning, spraying fuel from the
ruins of its port wing.  Both aircraft dived into the ground.

The command pilot of AV/2, Al White (North American's test pilot), ejected
and survived, although he suffered back injuries when the air bag that
should have served as shock absorber on his ejection capsule failed.  He
said later that the other pilot, Major Carl Cross (USAF), was slumped
forward in his seat; White tried to activate Cross's ejection capsule but
was unable to do so before he had to eject himself.  It was speculated that
Cross had suffered a blow to the head, or was incapacitated by G forces.
The F-104N pilot, Joseph A Walker (NASA's chief research pilot) was
apparently killed in the initial collision.

The collision is believed to have been caused by the F-104 getting too
close to the XB-70 and getting caught in its wingtip vortices, but nobody
knows for certain.

The surviving XB-70, AV/1 (tail number 20001), is on display at the Air
Force Museum at Wright-Patterson AFB, Dayton, Ohio.

[Information from _Valkyrie -- North American XB-70_ by Steve Pace (thanks
Dan); also thanks to Al Bowers for additional details]