The Boeing 720 is a retired American narrow-body airliner produced by Boeing Commercial Airplanes. Announced in July 1957 as a 707 derivative for shorter flights from shorter runways, the 720 first flew on November 23, 1959. Its type certificate was issued on June 30, 1960, and it entered service with United Airlines on July 5, 1960. A total of 154 Boeing 720s and 720Bs were built; production ended in 1967. As a derivative, the 720 had low development costs, allowing profitability despite relatively few sales.

Compared to the 707-120, it has a length reduced by 8.33 feet (2.54 m), a modified wing and a lightened airframe for a lower maximum takeoff weight. Originally designed to be powered by four Pratt & Whitney JT3C turbojets, the initial 720 could cover a 2,800-nautical-mile [nmi] (5,200 km; 3,200 mi) range with 131 passengers in two classes. The reconfigured 720B, powered by JT3D turbofans, first flew on October 6, 1960, and entered service in March 1961. It could seat 156 passengers in one class over a 3,200-nautical-mile [nmi] (5,900 km; 3,700 mi) range. Some 720s were later converted to the 720B specification. It was succeeded by the Boeing 727 trijet.

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 707-120. 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 707-020, then 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.

Because the aircraft systems were similar to the Boeing 707, no prototype Boeing 720 was built; any different systems were tested on the Boeing 367-80. The first 720 took its maiden flight on November 23, 1959. The type certificate for the 720 was issued on June 30, 1960. It first entered service with United Airlines on July 5, 1960; 65 of the original version were built.

The 720B version of the 720 had JT3D turbofan engines, producing 17,000 lbf (75.6 kN) each. The JT3D engines had lower fuel consumption and higher thrust. The maximum takeoff weight for the 720B was increased to 234,000 lb (106,000 kg). The 720B first took to the skies on October 6, 1960, and received certification and entered service with American Airlines in March 1961; 89 720Bs were built, in addition to conversions of American's 10 existing 720s.

As a modification of an existing model, the 720 had minimal research and development costs, which allowed it to be successful despite few sales. The company built 154 Boeing 720s and 720Bs from 1959 to 1967. The 720's wing modification was later added on the 707-120B and on 707-120s retrofitted to the B standard.^{[citation needed]}