Everybody is correct!
Boeing designed the 727 as a compromise to meet the requirements of United, American, and Eastern for a domestic aircraft that could operate out of shorter runways on medium routes to smaller airports.
United wanted a new aircraft for high density altitude airports; American wanted a twin engined aircraft for efficiency; and Eastern wanted a third engine for its Caribbean ETOPS requirements; and all three wanted shorter field capability. The first 727's even had nosewheel brakes!
Boeing designed the 727 with three rear mounted JT8D engines that increased the ETOPS of the time and allowed for a full uninterupted wingspan of the most advanced lift enhancing devices on a commercial aircraft at that time (Triple-slotted trailing edge flaps, Krueger flaps on the inner leading edge and slats on the outer). The third engine improved engine-out capabilities on the shorter runways and thus allowed greater takeoff weights.
It was also designed to operate independently of most ground support equipment and on gravel strips (Built-in rear airstair, APU, and reverse taxi capability).
Boeing 727 Family
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