That was also said about the B-707, which was supposed to have some parts commonality with their KC-135 Stratotanker built for the USAF. But as development progressed, the airliner and the tanker diverged.

The B-747 went through a similar process. Boeing was proposing a big cargo aircraft to the USAF (the CX-HLS), but that was never built. Lockheed got the C-5 contract instead, which satisfied the USAF's need for a really big cargo plane. So the B-747 was built as a commercial plane, mostly to Pan Am's requirements.

Military-civilian commonality was mostly wishful thinking at the management levels, as it turned out.

The 707 was redesigned with a wider fuselage to carry the number of passengers the airlines wanted.