BOEING 747SP (N531PA)

Didn't they think this could break mach one?

It's my understanding that in "normal testing" for the B-747 family, there was an incident where the queen of the sky was pushed to red line and actually broke through Mach 1. Although that was not the intent of the high speed run, it was recorded and caused a couple of impressive booms around the Seattle area. The B-747 was never designed to fly supersonic - too much mass to push through too high a drag curve.

We used to fly PAN AM to Greece back then. These were the birds we were on and on Olympic Airways too.

Good old pan am
And the faithful queen of the Sky

Chris, I know a 727 in a shallow dive could easily break Mach 1, and I've heard the 747 can as well.
like Ken said, not recommended but built to survive it.

Good old memory!