That last one was during performance testing by Douglas. I believe the forward part of the airframe was salvaged and mated with an aft fuselage (also recovered from another 'oops') and the resultant hybrid/frankenplane became the UHB testbed at one point.
Incidentally, the rules for how one tested to determine performance data were changed after that incident, to stop companies pushing the envelope too enthusiastically in pursuit of theoretical data.