United launches Boeing 720 Jet Mainliner service, introducing its second jet aircraft after the DC-8 entered service the previous year.  The B720 was able to land on shorter runways and more efficiently fly medium-length routes.  United ordered 11 of the B720s in November 1957, increased its order to 18 in June 1959, and took delivery of the first aircraft April 30, 1960.  The company flew the B720 until 1972.