Designed by Jack Northrop and Ed Heinemann as a private venture by Douglas, interest in the A-20 Havoc was soon shown by the French and British air forces shortly before the war and a number served in France during the Blitzkrieg after which the rest of the order was taken over by the RAF where the aircraft was known as the Boston. In RAF service, it was used extensively in the early and middle years of the conflict, mostly in the Mediterranean and North Africa and with the USAAF served in a variety roles including as a night fighter in the Pacific before the introduction of the P-61, but was most used as a highly maneuverable light bomber of which it was the most widely produced of its kind in the United States. With the advent of Lend Lease, 3,125 A-20s were sent to the Soviet Union (42% of all units) where they were used for close support and anti-shipping strike, others were used by Australia in the Pacific. Despite its pre-war design, A-20s served in the front lines until the last day of the war (although partly replaced by the A-26) and was also used by Brazil, Canada, the Netherlands, South Africa, and the UK.

The prototype, designated DB-7, was first flown on 17 August 1939 and initial production aircraft with the same designation went to the French and British. RAF versions were named Boston Mk. I (DB-7), Mk. II (DB-7A) and Mk. III (DB-7B) and the first variant that equipped the USAAF was the A-20A with turbocharged R-2600-7 radial engines. Minor structural improvements resulted in the A-20B (lighter and unarmored) and the A-20C (self-sealing fuel tanks) which was the first to see combat with the USAAF. A second-generation of aircraft began with the most produced variant, the A-20G which featured a solid nose housing six .50-in machine guns. Various final variants were also used including the A-20J which returned to a glazed nose configuration. Improved R-2600-29 engines resulted in the A-20H and the A-20K which were otherwise similar to the G and J, respectively. RAF versions of the A-20K and J were the Mk. IV and Mk. V.