The B 707's did well on all the routes they flew, but there was a market for a smaller Jet than the B707 and the DC-8 program. So Boeing launched a new airliner the B 720, a not well known aircraft, because of its relative short live before the B 727 was introduced into service in the mid 60's. So Boeing shorted the fuselage 16 ft and the span 15ft and the B 720 was born. The launching customer was United. Pan Am had only nine of them because of the B 727 Pan Am bought. This aircraft take much of the routes the B 720 was designated to serve.