Beech has produced six different basic King Air models, with the 90 series being the smallest.
The first Model 90 King Air was introduced in 1964; it was basically a Queen Air with turboprops. After its introduction, the 90 series went through a number of airframe and powerplant modifications. The original Model 90 was powered by a Pratt & Whitney PT6 engine.
In 1976, the C90 received a new engine, the PT6A-21. Another increase in cabin pressure (to 5 psi), a 15-knot cruise speed increase (due to an increase in the interstage turbine temperature [ITT]), a stronger horizontal stabilizer, and a new cabin door (borrowed from the King Air 200) was introduced in 1982 on the C90-1. The C90A brought redesigned engine cowlings that reduced drag and boosted ram airflow to the engines (resulting in a 12-knot increase in cruise speed), an electrical landing gear system, and a rudder boost system that eased the pilot workload during single-engine operations. Also included on this model (due to concerns about wing strength and cracking wing bolts) was a bonded spar cap, running the length of the wing using shear fittings instead of bolts.