[2][4][6][7] 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.

[13] Production got an early boost when Nationalist China paid the company US$750,000 for six M18R light bombers,[14] but by the time of the U.S. entry into World War II, only 39 Model 18s had been sold, of which 29 were for civilian customers.

[9][15] Work began in earnest on a variant specifically for training United States Army Air Forces (USAAF) military pilots, bombardiers, and navigators.

[16] The first JRB-1 obtained by the Navy, bureau number (BuNo) 09771, was converted from the last civil Model 18 built before production was earmarked solely for the military for the duration of the war.

[19] From 1951 to 1955, the USAF had many of its aircraft remanufactured with new fuselages, wing center sections, and undercarriages to take advantage of the improvements to the civil models since the end of World War II.

[20][21] With the adoption of the 1962 United States Tri-Service aircraft designation system, the Navy's SNB-5 and SNB-5P became the TC-45J and RC-45J respectively, later becoming the UC-45J as their primary mission shifted from aircrew training to utility transport work.

In later years, the military called these aircraft "bug smashers" in reference to their extensive use supplying mandatory flight hours for desk-bound aviators in the Pentagon.

[25] Beech 18s were used extensively by Air America during the Vietnam War; initially more-or-less standard ex-military C-45 examples were used, but then the airline had 12 aircraft modified by Conrad Conversions in 1963 and 1964 to increase performance and load-carrying capacity.

The configuration of the tubes in combination with drilled holes from aftermarket STC modifications on some of these aircraft have allowed the spar to become susceptible to corrosion and cracking while in service.

The corrosion on unmodified spars was not a problem; it occurred due to the additional exposed surface area created through the STC hole-drilling process.

The Beechcraft Model 18 family has been involved in the following notable accidents and incidents: Data from Jane's Fighting Aircraft of World War II.