In British Commonwealth air forces, bomber/attack variants of the DB-7 were usually known by the service nameBoston, while night fighter and intruder variants were usually known as Havoc. An exception to this was the Royal Australian Air Force, which referred to all variants of the DB-7 by the name Boston. The USAAF referred to night fighter variants as P-70.
In March 1937, a design team headed by Donald Douglas, Jack Northrop and Ed Heinemann produced a proposal for a light bomber powered by a pair of 450 hp (336 kW) Pratt & Whitney R-985 Wasp Junior radial engines mounted on a high-mounted wing. It was estimated that it could carry a 1,000 lb (454 kg) bomb load at 250 mph (400 km/h). Reports of aircraft performance from the Spanish Civil War indicated that this design would be seriously underpowered, and it was subsequently cancelled.
The model did, however, attract the attention of a French Purchasing Commission visiting the USA. The Neutrality Act of 1935 at the time forbade the sale of arms, including aircraft, to any nation at war, and President Roosevelt had just issued a call both for its revision and a rearmament program for the Air Corps. Aided by the Treasury Department's Procurement Division (headed by retired Naval officers) and Secretary of the TreasuryHenry Morgenthau, Jr., the French discreetly participated in the flight trials, so as not to attract criticism from American isolationists. The Air Corps, which controlled the aircraft's development but had been excluded from negotiations between the French, the Production Division, and the Navy's Bureau of Aeronautics, was directed by the White House on 19 January 1939 to release the DB-7 for assessment in contradiction of its own regulations. The "secret" was blown when the Model 7B crashed on 23 January while demonstrating single-engine performance. The French were still impressed enough to order 100 production aircraft, with the order increased to 270 when the war began. Sixteen of those had been ordered by Belgium for its Aviation Militaire.
Although not the fastest or longest-ranged in its class, the Douglas DB-7 series distinguished itself as a tough, dependable combat aircraft with an excellent reputation because of its speed and maneuverability. In a report to the British Aeroplane and Armament Experimental Establishment (AAEE) at RAF Boscombe Down, test pilots summed it up as "has no vices and is very easy to takeoff and land... The aeroplane represents a definite advantage in the design of flying controls... extremely pleasant to fly and manoeuvre." Ex-pilots often consider it their favorite aircraft of the war due to the ability to toss it around like a fighter. Its true impact was that the Douglas bomber/night fighter was extremely adaptable and found a role in every combat theater of the war and excelled as a true "pilot's aeroplane."
When DB-7 series production finally ended on 20 September 1944, a total of 7,098 had been built by Douglas and a further 380 by Boeing.
Douglas A-20J-10-DO, 43-10129, of the 416th Bomb Group destroyed by flak over Beauvoir, France on 12 May 1944.
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, but saw practically no action against the Allies except briefly during the Allied invasion of North Africa. 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. 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 Holland. 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. Actually the Soviet Air Force had more A-20 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 Browning machine guns and replaced them with faster-firing ShKAS. During the summer 1942, the Bostons flew 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 angle of up to 65° while the tricycle landing gear facilitated take-off and landings. The type could be flown even by scarcely trained crews. The engines were reliable but rather sensitive to low temperature, 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 USSR, 2,771 of which were used by the Soviet Air Force. 
Servicing an A-20 bomber, Langley Field, Va., July 1942
A-20 leaves the assembly line at the Long Beach, 1942
Boston I & II
The Royal Air Force agreed to take up the balance of the now-frustrated French order which was diverted to the UK, and the bombers were given the service name "Boston" with the further designation of "Mark I" or "Mark II" according to the earlier or later engine type.
The aircraft was generally unsuitable for use by the RAF since its range was too limited for daylight raids on Germany. Many of the Boston Mk II, plus some re-engined Mk Is, were converted for nighttime duties - either as intruders with 2,400 lb (1,100 kg) of bombs, or as night fighters with AI Mk IV radar. These were known collectively as Havoc Mk I. A total of 181 Bostons were converted to Havocs. In interdiction raids, Havoc intruders caused considerable damage to German targets.
Twenty Havocs were converted into "intruder" aircraft, carrying the Long Aerial Mine (LAM), an explosive charge trailed on a long cable in the path of enemy aircraft in the hope of scoring a hit. Trials conducted with lone Handley Page Harrows dropping LAMs into the stream of German bombers were not successful, and the Havocs were converted back to Mk I intruders.
Havoc fitted with a 2,700 million candela searchlight in the nose; the batteries for it carried in the bomb bay. A radar operator sat in the after fuselage. They were unarmed, and they were supposed to illuminate targets for accompanying Hawker Hurricane fighters, but in practice the conspicuous light made them easy targets for German gunners. A total of 31 aircraft were converted.
DB-7A / Havoc II
The French Purchasing Commission ordered 200 more bombers, to be fitted with 1,600 hp (1,195 kW) Wright R-2600-A5B Twin Cyclone engines. These were designated as the DB-7A by Douglas Aircraft. None of these were delivered before the fall of France, and hence they were sent to the UK instead. These were converted into night fighters by the addition of 12 0.303 inch machine guns in their noses and extra fuel tanks. The had a top speed of 344 mph (550 km/h) at higher altitudes. A total of 39 aircraft were used briefly as Turbinlites.
DB-7B / Boston III
The DB-7B was the first batch of this model to be ordered directly by the Royal Air Force. This was done in February 1940. These were powered by the same engines as the DB-7A, with better armor protection. Importantly, these had larger fuel tanks and they were suitable for use by the RAF as light bombers. This was the batch for which the name "Boston" was first assigned, but since the DB-7s intended for France entered service in the RAF first, the aircraft in this order were called the Boston Mk III. Among other combat missions, they took part in the attacks on the German warships Scharnhorst, Gneisenau, and Prinz Eugen during their dash through the English Channel (Operation Cerberus) and the raid on Dieppe ("Operation Jubilee"). Three hundred Boston III were produced and delivered and some of them were converted for use as night fighters.
A variation on the DB-7B/Boston III built for a French government order and featuring French instruments and secondary equipment; of the 480 DB-73s ordered by France, 240 were built by under license by the Boeing Company in Seattle. None were delivered, due to the fall of France, the DB-73 block was ordered by the RAF, after conversion to the Boston III configuration. However, following the German invasion of the Soviet Union, 151 DB-73s were provided to the USSR. Following the Japanese Attack on Pearl Harbor, a further 356 DB-73s were taken up by the USAAF, which transferred 22 to the Royal Australian Air Force (RAAF) for use in the South West Pacific theatre. Australian sources usually list these aircraft as DB-7B.
This was a Dutch Air Force version intended for service in the Dutch East Indies, but the Japanese conquest of the East Indies was complete before they were delivered. Part of this order was stranded in Australia in the so-called "lost convoy", and the first 31 Bostons were assembled at Richmond Airbase in New South Wales and flown by the 22 Squadron of the RAAF during the campaign against Buna, Gona, and Lae, New Guinea. The assembly of these 31 bombers was hampered by the fact that their manuals and instrument panels were printed in Dutch. The rest of this order were sent to the Soviet Union which received 3,125 of the Douglas DB-7 series.
T30 triple launcher for 4.5 in (114 mm) rockets, which were also carried by P-47s.
When shipments to the UK finally resumed, they were delivered under the terms of the Lend-Lease program. These aircraft were actually refitted A-20Cs known as the Boston IIIA.
The original American indifference to the Model 7B was overcome by the improvements made for the French and British, and the United States Army Air Corps ordered two models, the A-20 for high-altitude bombing and the A-20A for low and medium altitude combat. Both were similar to the DB-7B. The A-20 was to be fitted with turbosupercharged Wright R-2600-7 engines, but these were bulky and the prototype suffered cooling problems, so the remainder were completed with the two-stage supercharged R-2600-11, 59 as P-70 fighters and 3 as F-3 reconnaissance aircraft. One A-20 was evaluated by the U.S. Navy as the BD-1, while the U.S. Marine Corps flew eight as the BD-2.
The U.S. Army ordered 123 A-20As with R-2600-3 engines, and 20 more with the more powerful R-2600-11. They entered service in the spring 1941. The Army liked the A-20A because of its good performance and because it had no adverse handling characteristics. Nine of them were transferred to the RAAF in 1943. The USAAF used the British name Havoc for the A-20A, while the RAAF referred them as Bostons.
The A-20B received the first really large order from the Army Air Corps: 999 aircraft. These resembled the DB-7A rather than the DB-7B, with light armor and stepped rather than slanted glazing in their noses. In practice, 665 of these were exported to the Soviet Union, so only about one-third of them few served with the USAAF.
A-20C being serviced at Langley Field, Virginia, 1942.
The A-20C was an attempt to develop a standard, international version of the DB-7/A-20/Boston, produced from 1941. It reverted to the slanting nose glass, and it had RF-2600-23 engines, self-sealing fuel tanks, and additional protective armor. These were equipped to carry an external 2,000 lb (907 kg) aerial torpedo. A total of 948 were built for Britain and the Soviet Union, but many were retained by the USAAF after the Japanese attack on Pearl Harbor. The Soviet A-20s were often fitted out with turrets of indigenous design.
A-20G Havoc displayed at the National Museum of the U.S. Air Force.
The A-20G, delivered from February 1943, would be the most produced of all the series - 2850 were built. The glazed nose was replaced by a solid nose containing four 20 mm (.79 in) Hispano cannon and two .50 in M2 Browning machine guns, making the aircraft slightly longer than previous versions. After the first batch of 250, the unreliable cannon were replaced by more machine guns. Some had a wider fuselage to accommodate a power driven gun turret. Many A-20Gs were delivered to the Soviet Union. The powerplant was the 1,600 hp (1,200 kW) R-2600-23. US A-20Gs were used on low-level sorties in the New Guinea theatre.
The A-20H was the same as A-20G, continued with the 1,700 hp (1,270 kW) R-2600-29. 412 of these were built. The takeoff weight was raised to 24,170 lb (10,960 kg).
In 1948, the last surviving A-20H in United States service was redesignated "B-20" with the elimination of the 'A for Attack' category, and was given the "Z" prefix as being obsolete.
A-20J / Boston IV
The A-20J carried an additional bombardier in an extended acrylic glass nose section. These were intended to lead bombing formations, with the following standard A-20s dropping their bombs when signaled by the leader. A total of 450 were built, 169 for the RAF which designated them Boston Mk IV from the summer of 1944 onwards.
A-20K / Boston V
The A-20K (Boston Mk V in RAF parlance) was the final production version of the A-20 series, the same as the A-20J except for R-2600-29s instead of -23s.
In October 1940, the USAAC felt a need for long-range fighters more than attack bombers, sixty of the production run of A-20s were converted to P-70 night fighters, all delivered by September 1942. They were equipped with SCR-540 radar (a copy of the British AI Mk IV), the glazed nose often painted black to reduce glare and hide the details of the radar set, and had four 20 mm (.79 in) forward-firing cannon, each provided with 120 rounds, in a tray in the lower part of the bomb bay, while the upper part held an additional 250 gal (946 ltr) fuel tank. In 1943, between June and October, 13 A-20Cs and 51 A-20Gs were converted to P-70A. Differences were to be found in the armament, with the 20mm cannon package replaced by an A-20G gun nose with six .50 caliber guns installed, the SCR-540 radar installation being carried in the bomb bay with the transmitting antenna protruding between the nose guns. Further P-70 variants were produced from A-20G and J variants. The singular airframe P-70B-1 (converted from an A-20G) and subsequent P-70B-2s (converted from A-20Gs and Js) had American centimetric radar (SCR-720 or SCR-729) fitted. The P-70s and P-70As saw combat only in the Pacific during World War II and only with the USAAF. The P-70B-1 and P-70B-2 aircraft never saw combat but served as night fighter aircrew trainers in the US in Florida and later in California. All P-70s were retired from service by 1945.
The F-3A was a conversion of forty-six A-20J and K models for night-time photographic reconnaissance (F-3 were three conversions of the original A-20). This variant was employed in the European Theater by the 155th Photo Reconnaissance Squadron which began its deployment as the 423rd Night Fighter Squadron. The 423rd was converted to its photo mission as the 155th Photo Reconnaissance Squadron in part because of knowledge of night fighter tactics which could be used to defend against German aircraft. Although the armament was removed, the crew of three was retained, consisting of a pilot, observer, and navigator. The first Allied aircraft to land at Itazuke, Japan after the August 1945 surrender was an F-3A.
The BD-1 in 1940
One A-20A was bought in 1940 by the United States Navy for evaluation for use by the United States Marine Corps. The Navy/Marine Corps did not have any priority on the production lines, so the DB was not put into service.
In 1942, eight former Army A-20Bs were diverted to the United States Navy for use as high-speed target tugs. Despite the addition of the target-towing equipment and the removal of all armament and the provision to carry bombs, the aircraft were still designated BD in the Bomber sequence. They were withdrawn from service in 1946.
An observation/reconnaissance version of the A-20B powered by two 1,700 hp (1,268 kW) R-2600-7 engines. The original order for 1,489 aircraft was canceled and none were built.
Mesko, Jim. A-20 Havoc in action. Carrollton, Texas: Squadron/Signal Publications, 1983. ISBN 0-89747-131-8.
Taylor, John W.R. "Douglas DB-7, A-20 Havoc, and Boston (Bombers) and Douglas DB-7, Havoc, P-70 (Fighters)." Combat Aircraft of the World from 1909 to the present. New York: G.P. Putnam's Sons, 1969. ISBN 0-425-03633-2.
Thompson, Scott. Douglas Havoc and Boston: The DB-7/A-20 Series (Crowood Aviation Series). Ramsbury, UK: The Crowood Press, 2004. ISBN 978-1861266705.
Winchester, Jim, ed. "Douglas A-20 Boston/Havoc." Aircraft of World War II (The Aviation Factfile). Kent, UK: Grange Books plc, 2004. ISBN 1-84013-639-1.