The Beech Model 90 King Air was the first model in the largest and most successful family of corporate turboprop twins yet built. It began life as a turboprop development of the Queen Air, designed to meet an Army requirement for staff/utility transport. The company went on to produce another two civil models – both pressurized – before production ceased after 34 years following 2981 deliveries from 1971 onwards (the last six being delivered by Raytheon).

A prototype PT6 powered Queen Air Model 65-80 (later 65-90T) began test flying in 1963 and this type was subsequently ordered by the US Army in unpressurised form as U21A without any significant changes apart from increased capacity fuel tanks;