Boeing 307, Boeing 377 and the list goes on and BTW the original "707" prototype was actaully called the Dash 80.....
It was actually called the "367-80"...using the 300-series designator to mask the fact that it was a jet transport from the rest of the aviation industry.
I've heard in a few places that they stubled upon 707 as the SIN of 45 degrees (the wing angle of the 707 - what I heard) is .707...! I don't know this to be 100% true but I've heard it in a few places.