Boeing 727
The Boeing 727 was made to fly from small cities which had airports with shorter runways. It also carried fewer passengers.  United Airlines wanted a plane with four engines so that it could fly to airports which were at high altitude. An example of an airport like this is Stapleton International Airport in Denver, Colorado. American Airlines wanted a plane with two engines. Eastern Airlines wanted a plane with three engines so that it could fly to the Caribbean. The airlines decided to ask for a trijet, which is a plane with three engines.