Um. Pan Am expressed an interest in a large aircraft in 1965. In 1966, Boeing proposed a design, and agreed to deliver it by 1970. It entered service in December, 1969.
Come to think of it, I can't find any cases of major delays in Boeing's bringing a completely new commercial airframe to market. Now, delays undoubtedly do occur, such as the 737 backlog a few years ago, but on a new aircraft, on which the company is banking everything and concerning which there is more than a little debate about the utility or demand, delays are poison. Hell, if I'm buying aircraft, I'd be seriously considering an A380, since they'll be going at fire-sale prices.