The 727 short to medium range trijet is the world's second most successful jet airliner built. Initial design studies began in 1956, although for a time it appeared that a new short/medium range airliner would not be built at all due to Boeing's financial position before sales of the 707 had taken off. Boeing persisted however and serious development of the 727 beginning in June 1959. The program was launched on the strength of orders for 80 from Eastern and United in 1960.
The resulting Boeing Model 727 pioneered the rear trijet configuration, with power from three specially designed Pratt & Whitney JT8D turbofans (although RollsRoyce Speys were originally considered). The trijet design was settled upon as it gave the redundancy of three engines, better climb performance than a twin and improved operating economics over a four engine jet. The 727 also introduced an advanced wing design with the first airliner application of triple slotted Krueger flaps. The 727 retained the 707's fuselage cross section, but with a redesigned smaller lower fuselage due to the need to carry less baggage on shorter range flights, and it has limited parts commonality with the 707 and 720. The 727 was also the first Boeing airliner to feature an APU (auxiliary power unit).