Dubbed Model 18, the twin beech was produced in anticipation of an increase in demand from the military. With twin radial engines, a tailwheel undercarriage, and an all-metal construction, it was a fairly conventional design for the times--closely resembling the Lockheed Electra, for example. It was unique, however, in its twin tailfin configurations and multiple modifications, including a variety of engines and airframe alterations to change its weight and speed. Several different engines, however, were fitted once it was found that the engine mounts were its weakest structural area. By the late 1950s, the twin beech's modifications included taller fuselage for an added 6 inches of headroom, longer nose, and tricycle nosewheel and undercarriages; a factory option for more power was even available. Some required modifications, however, forced some models to retire. In 1975, one of those changes on the wing spar made it vulnerable to corrosion and cracking. The FAA eventually issued an Airworthiness Directive for the addition of a spar strap. The modifications, in the end, were more costly than the aircraft itself for some owners.
WWII skyrocketed sales from a surge in wartime contracts and a drop in competition as Lockheed moved its focus towards larger aircrafts. Twin beeches were used extensively during the Vietnam War by Air America and, by then, had its undercarriage doors redesigned and improved wingtips for smoother aerodynamics. In general, they have been used for a wide variety of both military and civilian use. Spraying, seeding--from fish to cloud--firefighting, air mail, skydiving, to even sterile insect releasing. It's still being used privately today, illustrating how useful the Beechcraft Model 18 is.