Wizofoz:
Douglas were proposing a twin variant of the DC 10, but figured the restrictions on twins would make it non-viable. Enter the B757/767 and ETOPS.... We'd probably still have a "Big Three" if they had gone ahead with it!
Not quite so simple. The DC-9 had two crew and two engines back in early 60s. Boeing had more of a struggle with the early 737 - designed for two crew/two engines, but UAL's union forced a third man into that cockpit.
Douglas's big twin was simply late to the game.