In 1934, the Army Air Corps put out a request for a bomber with double the bomb load and range of the Martin B-10, then the USAAC's standard bomber. In the evaluation at Wright Field the following year (where Boeing demonstrated its Model 299 that became the B-17 Flying Fortress), Douglas showed its Douglas DB-1. The Douglas design was ordered into immediate production in January 1936 as the B-18.
The DB-1 design was basically the same as the DC-2, but the wingspan was an additional 4.5 ft, and with a a mid-wing instead of a low-wing position on a deeper fuselage, so as accommodate both bombs and 6-member crew. Armament included dorsal and nose turrets, plus a ventral gun. The engines were two Wright R-1820-45 Cyclone 9s, developing 930 hp (694 kW) each.
The initial contract called for 133 B-18s (including the single prototype DB-1), using Wright radial engines. The last B-18 of the run had a power-operated nose turret and was designated DB-2 by the company, but did not become standard. Additional contracts in 1937 (177 aircraft) and 1938 (40) were for the B-18A, which has the bomb-aimer's position further forward (over the nose-gunner's station), and Wright R-1820-53 Cyclone 9 engines (1,000 hp, 746 kW).
B-17s supplanted B-18s in first-line service in 1942. 122 B-18As were equipped with search radar and magnetic anomaly detection (MAD) equipment, designated B-18B, and used in the Caribbean on anti-submarine patrol. The RCAF acquired another 20, called them the "Douglas Digby Mk I", and used them for patrols also.
The B-22 was a proposed followon using Wright R-2600-3 radial engines, but it was never built.