Like many aircraft of this era, the B-18 was developed from a civilian airplane. In this case, the influence was Douglas Aircraft Corporation's DC-2 Airliner (which also was a major influence in the development of the iconic DC-3/C-47). It first flew in April of 1935 and was tested under the designation DB-1, in competition with the Martin Model 146 and the Boeing Model 299 (the latter of which would eventually become the famous B-17 Flying Fortress).

The Martin Model 164 was basically a larger version of the Glenn L. Martin Company's earlier design, the aforementioned B-10. This was not a strong selling point for their prototype, as the USAAC wanted a new design without any of the B-10's percieved limitations and problems. The Boeing Model 299 was seen as an excellent aircraft and slated to win the contract, but there were two factors that led to its demise in this competition: First, the US was in the middle of the Great Depression, making an expensive aircraft undesirable. Second, during a test flight on 30 October 1935, the sole prototype crashed when the crew forgot to disengage a set of control surface locks called 'gust locks.'

Due to this, Douglas Aircraft Corporation won the contract, receiving an initial order of 133 B-18 Bolos. By 1940, most USAAC bomber squadrons were equipped with some variant thereof. However, in this era, aeroplane technology was evolving rapidly; by the time the attack on Pearl Harbor occurred in 1941, the B-18 was outdated and it was hoped that it would at least be a sufficient stopgap until the more advanced B-17 and B-24 bombers became widely available.

While some B-18s were stationed overseas in the Philippines when America entered WW2, these bombers were mostly used for mainland defence of the USA, for bomber crew training, or as transport aircraft. On 22 August 1942, a B-18 Bolo piloted by Lt. Koenig of the 45th Bomber Squadron sunk the German submarine U-654 in the Caribbean. This was the first time a U-boat had been sunk by US aircraft (Source). Additionally, one particular B-18 was outfitted with a 75mm cannon for testing (Another photo). While this experiment proved to be a failure, it provided important data for the installation of a similarly sized cannon in the B-25.

The B-18 Bolo also saw service with the Royal Canadian Air Force (where it was nicknamed the 'Digby') and the Brazilian Air Force. By the end of WW2 in 1945, the B-18 had been completely phased out of USAAC and RCAF service. In 1946, the Brazilian Air Force retired their B-18s.

The 21st Observation Squadron, now known as the 911th Air Refueling Squadron, is one of the oldest squadrons in the U.S. Air Force. It was first formed in May of 1917 at Kelly Field, Texas as the 21st Aero Squadron and served in France during WW1 as part of the American Expeditionary Force. In 1923 it was formed as the 21st Observation Squadron but was disbanded in 1933. Two years later at Langley Field, Virginia, a new 21st Observation Squadron was formed. In 1939 they participated in Neutrality Patrol missions around the Florida coast and Atlantic, and upon the USA joining WW2 flew anti-submarine patrols. The unit also served as a heavy bomber training group, and upon being equipped with the B-29 Superfortress in 1944, began to fly bombing missions over Japan. In 1958 they were reformed as the 911th Air Refueling Squadron and received KC-135A Stratotankers. They continued to serve as an air refueling squadron until being deactivated for less than a year in June 2007. In April 2008, the 911th Air Refueling Squadron was reactivated at Seymour Johnson Air Force Base in North Carolina.

This particular kit is the Special Hobby 1:72 B-18 'Pre War Service' kit, which was released in 2012. The particular livery is that of a B-18 stationed with the 21st Observation Squadron at Langley Field, Virginia in 1938. An imgur album of the scale model and reference photo is available here.