50-year old 707s with very high flight-hours. What could go wrong?
They got this ass backwards. The KC-135 came first and the 707 "airliner" was born as a spinoff of that project. They were both preceded by the dash 80 prototype.
The 707 was designed with a wider fuselage to accommodate 6 across seating. The KC-135 was designed to make passengers uncomfortable while sitting in the dark over hundreds of gallons of JP-4.