This seems like an odd version of the story. My understanding is Boeing designed lots of military aircraft, not all reached production. The 747 is the result of some of that design work.

You should read the book, if you're interested. From the book (p.84):

"Time and time again there appears in print the logical but false assumption that Boeing took its losing military C-5 bid and revamped it as the commercial 747. In fact, the 747 would be an entirely original design that owes nothing to the C-5."

That said, in the same chapter he talks about how GE developed a high-bypass turbofan engine for the C-5 and it was only because they had such an engine that the 747 became possible.

But really my only point is that you should read the book if you're interested.

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.

That makes sense because a high cockpit also combines well with nose loading which some military freighters have.

My memory of the 747 was that it was originally the military who paid for the design. They wanted an aircraft that could be loaded from the front. This led to the bubble at the front of the plane. For whatever reason, the military didn't bite so they repurposed it as a commercial aircraft.