The French order called for substantial modifications, resulting in the DB-7 (for Douglas Bomber 7) variant. It had a narrower, deeper fuselage, 1,000 hp (746 kW) Pratt & Whitney R-1830-SC3-G radials, French-built guns, and metric instruments. Midway through the delivery phase, engines were switched to 1,100 hp (820 kW) Pratt & Whitney R-1830-S3C4-G. The French designation was DB-7 B-3 (the B-3 signifying "three-seat bomber").
The DB-7s were shipped in sections to Casablanca for assembly and service in France and French North Africa. When the Germans attacked France and the Low Countries on 10 May 1940, the 64 available DB-7s were deployed against the advancing Germans. Before the armistice they were evacuated to North Africa to avoid capture by German forces. Here, they fell under control of the Vichy government, briefly engaging the Allies during the Operation Torch.
After French forces in North Africa had sided with the Allies, DB-7s were used as trainers and were replaced in front line units by Martin B-26 Marauders. In early 1945, a few DB-7s were moved back to France where they saw action against the remaining isolated German pockets on the western coast.
The remainder of the order which was to have been delivered to France was instead taken up by the UK via the British Purchasing Commission. In the course of the war, 24 squadrons operated the Boston. It first entered service with RAF Bomber Command in 1941, equipping No. 88 Squadron. Their first operational use was not until February 1942 against enemy shipping. On 4 July 1942 United States Army Air Force (USAAF) bomber crews, flying RAF Boston aircraft, took part in operations in Europe for the first time attacking enemy airfields in the Netherlands. They replaced the Bristol Blenheims of No. 2 Group RAF for daylight operations against occupied Europe until replaced in turn by de Havilland Mosquitos. Some Havocs were converted to Turbinlite aircraft which replaced the nose position with a powerful searchlight. The Turbinlite aircraft would be brought onto an enemy fighter by ground radar control. The onboard radar operator would then direct the pilot until he could illuminate the enemy. At that point a Hawker Hurricane fighter accompanying the Turbinlite aircraft would make the attack. The Turbinlite squadrons were disbanded in early 1943.
Through Lend-Lease, Soviet forces received more than two-thirds of version A-20B planes manufactured and a significant portion of versions G and H. The A-20 was the most numerous foreign aircraft in the Soviet bomber inventory. The Soviet Air Force had more A-20s than the USAAF.
They were delivered via the ALSIB (Alaska-Siberia) air ferry route. The aircraft had its baptism of fire at the end of June 1942. The Soviets were dissatisfied with the four .30-calibre Browning machine guns — themselves only capable of firing at a top rate of 600 rounds per minute apiece — and replaced them with the faster-firing, 7.62mm calibre ShKAS, capable of up to an 1800 rounds per minute firing rate apiece. During the summer of 1942, the Bostons flew ultra low-level raids against German convoys heavily protected by flak. Attacks were made from altitudes right down to 33 ft (10 metres) and the air regiments suffered heavy losses. By mid-1943 Soviet pilots were well familiar with the A-20B and A-20C. The general opinion was that the aircraft was overpowered and therefore fast and agile. It could make steep turns with an angle of up to 65°, while the tricycle landing gear facilitated take-off and landings. The type could be flown even by crews with minimal training. The engines were reliable but sensitive to low temperatures, so the Soviet engineers developed special covers for keeping propeller hubs from freezing up.
Some of these aircraft were armed with fixed-forward cannons and found some success in the ground attack role.
By the end of the war, 3,414 A-20s had been delivered to the USSR, 2,771 of which were used by the Soviet Air Force.