By the late 1930s, Beechcraft management speculated that a demand would exist for a new design dubbed the Model 18, which would have a military application, and increased the main production facilities. The design was mainly conventional for the time, including twin radial engines, all-metal semimonocoque construction with fabric-covered control surfaces, and tailwheel undercarriage. Less conventional was the twin-tailfin configuration. The Model 18 can be mistaken for the larger Lockheed Electra series of airliners, which closely resemble it. Early production aircraft were powered either by two 330-hp (250-kW) Jacobs L-6s or 350-hp (260-kW) Wright R-760Es. The 450-hp (336-kW) Pratt & Whitney R-985 became the definitive engine from the prewar C18S onwards. The Beech 18 prototype first flew on 15 January 1937, and type certification followed on 4 March that year.

AG1 And Down-Trim for door :P