Originally Posted by barit1
The 367-80 story is really an extension of earlier Boeing designations.
Remember the Stratocruiser? The 377? Its military counterpart, the C-97/KC-97, had the company designation 367. There were several derivatives planned, and one turboprop version (C-97J) went into limited USAF service.
As it became clear that only a turbojet tanker would meet future needs, the 367 tanker/transport series went through several more paper cycles, until the final 367-80 bore no resemblance to the KC-97.
But when the USAF ordered the KC-135, the fuselage diamater grew slightly (4" IIRC) from the -80. The fuselage of the commercial model, the 707, was about 12" bigger than the -80.
And - the 720 was a marketing designation only. The nameplate and type certificate call the "720" a 707-0xx (where the xx is the Boeing customer code).
And 707 was not the first Boeing model to end with 7.
There had been models like Boeing 80 and Boeing 247. I suspect that omitting numbers was already established and Boeing did not have every single model from 81 to 246... anyway, there were also Boeing 307 and 367 and 377. But also non-7 models like Boeing 314.
After the 4xx, 5xx and 6xx ranges were reserved, Boeing seems to have chosen 707 rather than 700 or 704 etc. partly to keep good luck from the numbers like 247...