Skyhawk1:
The 747 started life as a cargo plane, and never had anything to do with pax evac tests. The 747 was always designed from the outset as only having a short upper deck - never full length. The reason was to move the cockpit up high so cargo could be loaded through the nose. It was the competition to the C-5 Galaxy. When Boeing lost the competition - they turned it into a passenger plane.
Not exactly right. There clearly is some influence from the C-5 proposal and some sharing of engineering talent but the aircraft are quite different. The main difference was the C-5 proposal had a high wing instead of a mid wing, and had landing gear and performance suited to landing on shorter, less developed airfields.
Clearly after the loss of the C-5 competition Boeing knew a large transport was feasible and also that they had the talent to produce it. But the design is really quite different.
As you probably know, the reason the 747 retained the attributes of a cargo aircraft (including the ability to load cargo through the nose) was Boeing believed that supersonic passenger planes would be available in a few more years and they needed a role for the 747 to perform once it was retired from passenger service.
--ev--