The background is:
- Britain sent data on the Miles M.52 supersonic jet to the US, so at least some Americans knew about the flying tail before the X-1 (first supersonic flight in 1948), and the Germans did some work on flying tails
https://en.wikipedia.org/wiki/Miles_M.52
- most of the pre X-1 planes that crashed didn't reach Mach 1, but they did lose control. Those are called loss of control due to compressibility, not supersonic flights.
- as the article says, the DC-8 flew ok up to Mach 0.97 in previous flights, so they thought things were fine. Note that in every Mach 1.0 flight that I've read about, the airspeed indicator jumps from around Mach 0.96 to 1.0, perhaps meaning the indicator is not accurate around Mach 1.0.