"Since the naming of the initial 707, all Boeing commercial jets have been named in succession based on the 7-7 formula: 717, 727, 737, 747, 757, 767 and 777 up to the latest Boeing commercial jet transport, the 787".
Boeing Press Release, 28th January 2005
The 720 fitted really neatly into that formula, didn't it?
Nothing like re-writing history to make everything seem logical and well thought-through.
