The Model 90 King Air was conceived as the Model 120 in 1961. In May 1963, Beechcraft began test flights of the proof-of-concept Model 87, a modified Queen Air with Pratt & Whitney Canada PT6A-6 engines. On 14 July, Beech announced a new type, and a month later began accepting orders for the "King Air", with deliveries to commence in Autumn 1964. After 10 months of test flying, in 1964 the Model 87 was delivered to the United States Army as the NU-8F. On 24 January, the first definitive prototype, by now designated Model 65-90 and also fitted with PT6A-6 engines, flew for the first time. The first production aircraft was delivered on 8 October, and by the end of the month ,152 aircraft had been ordered;[2] by year's end, seven had been built.

In 1966, after 112 65-90s were completed, production switched to the Model 65-A90 with PT6A-20 engines. As a measure of the type's popularity, 206 65-A90s were built in less than two years when production switched to the Model B90, the first of these rolling off the production line in 1968. Military versions built during these years included the 65-A90-1, 65-A90-2, 65-A90-3, and 65-A90-4, all being unpressurised models based on the Model 87. These were produced for the US Army which designated them U-21s of various sub-models; many were fitted out for electronic battlefield surveillance. A total of 162 of these were built between 1967 and 1971.