In the year 1934 the United States Army Air Corps sought for a twin engine bomber, with twice the payload and twice the range of the Martin B-10. In the year 1935 a fly-off on Wright Patterson Air Base was made. Douglas used its DB-1 for the competition. It competed with the Model 299 by Boeing, which was later developed into the B-17 Flying Fortress; and with the Model 146 by Martin, which was an export version of the B-10. Boeing had the best design, but because of a crash Boeing's prototype was terminated first. With a unit cost of $58,500, the DB-1 was cheaper than the B-17, which had a unit cost of $99,620, being really important at the time of the Great Depression. In January 1936, production of the B-18 was started.
The DB-1 was based on the Douglas DC-2 in most aspects, but with many modifications. The wingspan was 1.4 m greater. It had a deeper fuselage to make room for more bombs and the crew of six persons. The bomber was powered by two Wright R-1820-45 Cyclone 9-radial engines with a power of 694 kW each. In the first contract 133 B-18 (including DB-1) with Wright-engines were ordered. The last aircraft of the series was called DB-2 and was equipped with a power-operated nose turret. Additional contracts were made in 1937 in which 177 B-18A and in 1938 40 B-18A were ordered. It had an additional bombardier's position further forward over the nose-gunner's station and was equipped with more powerful Wright R-1820-53-radial engines with a power of 746 kW each. In 1940 most of the bomber squadrons were equipped with B-18 and B-18A bombers. Some of those bombers were destroyed on 7th December 1941 on the Attack on Pearl Harbour.
In 1942 the B-18s were replaced by the B-17 bombers. 122 B-18A were modified into anti-submarine aircraft. For this role they were equipped with a Magnetometer. Those aircraft were called B-18B and were used in the Caribbean Sea for anti-submarine warfare. Two aircraft were sent in 1942 to the Brazilian Air Force. The Royal Canadian Air Force bought 20 B-18A, which were called Douglas Digby Mark I. All in all the Bolos and Digbys were able to sink four submarines.
- B-18 - Standard early bomber version with 930 hp (694 kW) R-1820-45 Cyclone 9 raidial engines. 133 aircraft.
- B-18A - improved version with 1,000 hp (746 kW) R-1820-53 raidial engines and extended bomb aimer's position over the nose gunner's station. 217 aircraft.
- Digby Mk I - 20 B-18As passed to the Royal Canadian Air Force for maritime patrol.
- B-18AM - 18 B-18As modified for training by removal of their bombing equipment.
- B-18B - New designation applied to B-18As fitted with search radar and Magnetic Anomaly Detection (MAD) equipment for anti submarine patrol in the Caribbean. 122 aircraft
- B-18C - Designation for 2 aircraft modified for anti-submarine patrol duties.
- B-18M - 22 B-18s that were modified to the same standard as the B-18AMs
- B-22 - Projected B-18A development with 1,600 hp (1,193 kW) R-2600-3 radial engines.
- C-58 - Two aircraft converted for transport duties.