Boeing 720B

Boeing 720
Trans Polar Boeing 720 arriving at in June 1970
Role	Narrow-body jet airliner
National origin	United States
Manufacturer	Boeing Airplane Company
First flight	November 23, 1959
Introduction	July 5, 1960 with United Airlines
Retired	September 29, 2010
Status	Retired
Primary users	United Airlines Western Airlines Eastern Airlines Northwest Airlines
Produced	1958–1967
Number built	154
Developed from	Boeing 707

The Boeing 720 is a four-engine narrow-body short- to medium-range passenger jet airliner. Developed by Boeing in the late 1950s from the Boeing 707, the 720 has a shorter fuselage and less range. The 720 first flew in November 1959 and the model entered service with launch customer United Airlines in July 1960.

Two primary versions of the aircraft were built. The original 720 with Pratt & Whitney JT3C turbojet engines entered service in 1960, while the improved 720B with Pratt & Whitney JT3D turbofans entered service in 1961. Some 720s were later converted to 720B specification.

Although only 154 were built, the Boeing 720 and 720B were profitable due to the low research and development costs of their being slightly modified versions of the 707-120. They were later replaced by the Boeing 727. Some 720s are on display at the Utah Aviation Museum.

Boeing announced its plans to develop a new version of the 707 in July 1957. It was developed from the 707-120 to provide for short- to medium-range flights from shorter runways. The model was originally designated 707-020 before being changed to 720 at the input of United Airlines. Compared to the 707-120, it has four fewer frames in front of the wing and one fewer aft: a total length reduction of 8 feet 4 inches (2.54 m).

The new model was designed to a lower maximum takeoff weight with a modified wing and a lightened airframe. The wing modifications included Krueger flaps outboard of the outboard engines, lowering take-off and landing speeds—thus shortening runway length requirements—and a thickened inboard leading edge section, with a slightly greater sweep. This modification increased the top speed over the -120. It had four Pratt & Whitney JT3C-7 turbojet engines producing 12,500 lbf (55.6 kN) each.

At one point in the development phase, it was known as the 717-020, although this was the Boeing model designation of the KC-135 and remained unused for a commercial airliner until it was applied to the MD-95 following Boeing's merger with McDonnell Douglas in 1997.

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