In early 1963 Beech had started flight tests of an aircraft, then identified as the Beech Queen Air Model 65-80, which was powered by two 373kW Pratt & Whitney Aircraft of Canada turboprop engines. Then envisaged as a further extension of the Queen Air range, the type had been developed to satisfy a US Army requirement for a staff/utility transport. The above designation was a little confusing as there were Model 65 and Model 80 Queen Airs in production, so the aircraft became known temporarily as the Model 65-90T (T standing for turboprop). In due course, an even better job was made of clarifying the situation by renaming the turboprop-powered Queen Airs as King Airs. In effect, therefore, the Model 65-90T was the prototype for the Beech Model 90 King Air series, but more specifically became the prototype of the unpressurised military Kings Airs. Following the first flight of the Model 65'-90T, a civil equivalent was produced in parallel with a pressurised cabin, and the first production prototype of this aircraft, designated Model 90 King Air, flew for the first time on 20 January 1964. These aircraft represented the beginning of the King Air series, which now represents an important area of Beech activities, and the 3,000th example of the King Air family was delivered to a customer on 17 April 1981.

US Army testing of the Model 65-90T, under the military designation NU-8F, had shown the aircraft to be suitable for the military requirement, so an initial order for 48 aircraft, under the designation U-21A, was placed. Beech distinguished its military King Airs from civil versions by identifying them as Model 65-A90-1, and began modification of the civil aircraft to provide a utility interior. This accommodates a crew of two and 10 troops, or six to eight command personnel, or three stretchers, and seating can be removed easily for the carriage of up to 1361kg of cargo.

Initial deliveries of production U-21As, which were given the name Ute, began on 16 May 1967, and subsequent contracts resulted in more than 160 being built. These included U-21As and RU-21A/RU-21D variants all with 410kW Pratt & Whitney PT6A-20 turboprops, and RU-21B/ RU-21C/RU-21E variants with 462kW PT6A-29s. The RU-21s were developed especially for operation in an electronic reconnaissance role in South-East Asia, sprouting a strange collection of aerials and sensors, and being equipped internally with related avionics' systems, plus nav/com systems suitable for all-weather operations. RU-21Bs and RU-21Cs had Beech designations Model 65-A90-2 and Model 65-A90-3 respectively, and the designation U-21G applied to 17 aircraft for the USAF that were similar to the U-21A. Deliveries of the civil Model 90 King Air began in late 1964, this having cabin pressurisation, and accommodating a maximum of 10 persons, including the pilot. It was superseded in early 1966 by the King Air A90, which introduced the more powerful PT6A-20 engines, and one of these aircraft was supplied for military service under the designation VC-6A.

The A90 was followed by a King Air B90 with detail improvements, and in September 1970 by the King Air C90 which introduced a more advanced pressurisation and heating system for the cabin. The C90 remains the current production version in 1982, with a total close to 1,000 having been delivered by the end of the year. One of these, designated VC-6B, also serves with the USAF's 1,254th SAM Squadron. Since its introduction, the C90 has been the subject of steady improvement, and current powerplant is the PT6A-21. Ten examples of the C90 serve with the Spanish air force and civil aviation school for instrument training and liaison.

A first extension to the King Air 90 range came in the early summer of 1972 with the introduction of the King Air E90. Generally similar to its predecessor, it has more powerful PT6A-28 turbo-props, flat-rated to 410kW, and this version remains in production in 1990. In 1976 Beech was awarded a contract from the US Navy for an advanced pilot-training aircraft that combined features of the C90 and E90, designated T-44A.

The most recent model in the King Air family was the Model F90 Super King Air, of which deliveries began in mid-1979. This combined the pressurised fuselage of the Model 90, with the wings and tail unit of the Models 100 King Air and 200 Super King Air respectively. Power was provided by two PT6A-135 engines, these driving slow-turning four-bladed propellers, that gave a much quieter cabin environment. While production of the F90 ended at 231 aircraft, the C90 continues in production, 1,415 of the low-tailplane variants having been delivered by early 1989.