The Beechcraft King Air series has its roots in the Twin Bonanza of 1951. That aircraft model was enlarged and re-engined to become the Model 65 Queen Air. The Queen Air design changed to incorporate a swept tail and pressurization. With the addition of Pratt & Whitney turboprop engines, the Queen Air became the Model 90 King Air. The King Air 90 led to the stretched King Air 100.
The 100 and A100 have Pratt & Whitney PT6A-28 engines, while the B100 has Honeywell (Garrett) TPE 331-6 engines. The B100 offered more powerful engines over the A100 and was the only King Air produced without Pratt & Whitney engines. Neither model was as popular as the 200/B200 series. The 200/B200s have more payload and higher gross weights than the 100s. The 100s seat six and include aft-lavatories. The cabin is not round and is shaped more like a loaf of bread, which gives passengers more shoulder room.