On January 15, 1937, the Beechcraft Model 18 made its first demonstration flight at the factory in Wichita, Kansas, and it continued in production for thirty-two years. This low-wing, all-metal, twin-engine monoplane was originally intended as a six-to-eight-passenger executive or feeder airline transport. As the years passed, however, the Model 18 was adapted to many uses and, in all, thirty-two different versions were produced.

When production began on the Model 18 in 1937, there was virtually no market for this airplane in the United States. At the time, air transportation in the United States was a trunk line operation, and few feeder lines existed. Acceptance of the Model 18 by foreign and charter lines was immediate, however. The Model 18A, which also operated on interchangeable ski- or float-landing gear, was an ideal adaptation for snowbound areas and for lake and inter-island service. Prairie Airlines of Alberta, Canada, for example, ordered several of these airplanes for use in delivering air mail over a route that extended from Prince Albert to North Battleford, south to Saskatoon and Moose Jaw, finally joining up with the main route of Trans Canada Airlines at Regina. Also, businessmen were favourably impressed with the performance of the Model 18 as an executive transport, with orders coming from Alaska, Canada, and Puerto Rico.

On January 13, 1939, Beech began negotiations with the U.S. government on a contract for a photo reconnaissance version of the 18. Fourteen of these aircraft, designated Type F-2, were ordered as part of the Emergency Procurement Program. This order was followed by a contract for eleven C-45 personnel transports. Later that year, Beech began negotiations with the Chinese government for a bomber trainer. This version had a clear plastic nose, a single gun turret on the upper fuselage, and a machine gun in a tunnel in the rear floor. It also had internal bomb racks, which carried up to twenty 25-pound bombs.

1939 also saw a standard Beech 18S set a new flight record while on a demonstration tour, flying from Bogota to Barranquilla, Colombia, a distance of 450 miles, in 1 hour, 54 minutes. Later the same airplane made a 1,350-mile flight from Maracay, Venezuela, to Miami, Florida, in 6 hours, the first known non-stop flight between those two cities. To further demonstrate the capable performance of the Beech Model 18, Walter Beech entered a D18S in the 1940 Macfadden Race from St. Louis to Miami. With "Ding" Rankin as his pilot, Beech crossed the finish line in Miami in 4 hours, 37 minutes to win first place. Their average speed for the flight was 234 mph.

World War II brought more orders for military versions of the Beech 18S from the United States and foreign governments for a wide range of uses. About 90 percent of the U.S. Air Force's navigators and bombardiers received their training on AT-7s and AT-11s respectively. The U.S. Navy SNB-1 was similar to the AT-11, the SNB-2 to the AT-7. The JRB-1 was a radio-control airplane for target or drone aircraft. The Navy's personnel transports similar to the C-45 were known as JRB2, JRB-3, and JRB-4.

With the end of the war came the end of military production, although many of these aircraft remained in service for years. By October 1945 Beech was back into full commercial aircraft production. The first aircraft off the line was the newest model, the D18S, which incorporated a number of improvements. Structural modifications allowed for an increase in maximum weight and new landing gear, brakes, and tires were installed. Two 450-hp Pratt & Whitney Wasp, Jr., engines with Hamilton Standard constant speed propellers powered the D18S. It was the premier executive transport among businessmen and it was also used by the new local service airlines that emerged after the end of World War II.

On December 10, 1953, the prototype of the Super 18, the last version of the Beech 18, made its first flight. The last three production aircraft were delivered in November 1969. More than 9,000 Model 18s were produced since 1937, and, in 1970, more than 2,000 were still being flown in the United States alone.