Actually.. Juan Trippe of PanAm pushed the double-decker concept. Boeing didn't want to do it.
Joe Sutter, was the lead designer on the Boeing 747. From the Seattle PI:
"Juan Trippe was double-decker-happy," Sutter recalled.
Until the day Sutter invited Trippe and other Pan American executives to check out a double-decker mock-up Boeing had constructed.
Boeing boss Bill Allen took Trippe and his party up to the top of the mock-up. Sutter stayed below -- way below. Sutter wanted Trippe to try an emergency slide that had been set up from the upper deck. Trippe refused and quickly came back down the shaky stairs.
Trippe was then taken to single-deck mock-up with the wide cabin that would become the hallmark of the 747 interior.
"He walked into that wide single-deck mock-up, and he didn't say a word," Sutter recalled. "But you knew that was the way he wanted to go."
There was one last visit that day, to a mock-up of the cockpit, which would be situated above the main cabin. For aerodynamic reasons, Boeing had created a large empty space just behind the cockpit. It would become the 747's signature hump.
Sutter recalled that Trippe turned to John Borger, a Pan Am engineer, and asked what the space was for.
Borger replied that it could be used for crew rest, Sutter recalled.
"This will be for passengers," Trippe replied.
Sutter was against a full double-deck 747 design for two main reasons. He was worried about slide interference with the wings from the upper deck in case of an emergency evacuation. And the two-deck design would leave little room for the 747 to carry a lot of cargo in its belly.