United was more seriously interested in the Concorde, it appears to me. They'd been a Douglas company for a while, so that might have been a factor. But the main reason they did not invest in SST programs beyond study was the impossible economics, which Concordes later inability to generate sales proved. Also, they had no foreign routes in the 1960's so supersonic flight was not a viable option, and the Air Force was getting enough criticism already to convince airlines that sonic booms were not going to be acceptable over land.
The American SST designs which narrowed down to Lockheed and Boeing, and finally Boeing, were bigger, faster and more complicated than Concorde in terms of systems required for a normally trained crew to fly at Mach 3. And even more expensive.
I don't think the cause of the XB-70 crash in 1966 was more than bad luck, and it certainly didn't kill anyone on the ground, but my understanding is that United understood what a long and costly development program would be required to safely move 250-300 people at Mach 2 plus. And Sen. Proxmire certainly led the charge to throttle federal funding for the a/cs development.
Just my