The only (known) aircraft capable of sustained flight above Mach 3 — it didn’t fly *as* high as the U2, but could still get up pretty high; wasn’t stealthy in its last decade or so, but was for all practical purposes invisible for years. “What’s that?” “What?” “On the scr- oh, there’s nothing there, never mind.” Already mothballed — reportedly because the corrugated exotic-alloy skin had holes cut in it for expansion during flight, such that it leaked fuel during those minutes it sat on the runway before takeoff, and some base commanders were freaks about that — it was returned to service during the Gulf War because intel needs far exceeded the capability of the recon satellites then in orbit.
U-2 can get to about 70,000 ft, SR-71 would sustain 85,000 ft.