In 1940, the new Douglas bomber also attracted the attention of the British Purchasing Commission, which was, like its French counterpart, touring various American aircraft factories in search of combat planes. The British Purchasing Commission was impressed with the DB-7, but wanted some changes which would better suit it to British requirements.
An initial order for 150 was placed on February 20, 1940, which was increased to 300 on April 17. The British version of the Douglas bomber was designated DB-7B. As compared to the DB-7A for France, the DB-7B for Britain had revised systems and introduced a flat-glass bomb-aimer nose extending six inches further forward and having 25 percent more glazed area. Instead of a stepped arrangement for the nose glass, the glass went back at a diagonal angle for improved visibility. The DB-7B was fitted with British instruments and bomb racks and was armed with 0.303-inch machine guns. Power was provided by two 1600 hp Wright R-2600-A5B radials that were equipped with two-speed superchargers. As compared to the DB-7A, the self-sealing tanks were improved and armor protection was better. Total fuel capacity was increased from 205 US gallons to 394 US gallons in order to improve the range, which had been the primary weakness of the earlier Douglas bombers. Because the British 0.303-inch machine guns were larger than the French 7.5-mm guns, four could not be fitted into the lower nose, so two were kept inside and the other two were carried outside the nose in removeable streamlined blisters.
Following the standard RAF practice of assigning popular names to their aircraft, the name Boston was assigned to the DB-7B. Roman numerals were used to designate different versions. However, by the time that deliveries of the DB-7B to Britain had started, the designations Boston I and II had previously been applied to DB-7 aircraft commandeered from French orders, and so the DB-7Bs were designated Boston III. RAF serials were W8252 to W8401 and Z2155 to Z2304. The first DB-7B flew on January 10, 1941. 541 were built between May and December of 1941. In addition, one DB-7B (AH740) was delivered as a replacement for the DB-7A (AH430) which had crashed while under test in the USA.
Boston III serial number W8315 was experimentally fitted with a twin-Browning Bristol power-operated dorsal turret, and W8268/G was fitted with four rocket-launching rails underneath each wing. These options were not adopted as standard, but a Martin power turret later became standard on later A-20s for the USAAF.
The Boston III began to arrive in Britain in the spring of 1941. The Boston III was the first of the DB-7/A-20 series actually to operate with the RAF in its intended role of light bomber. They were supplied to Nos 88, 107, 226, and 342 Squadrons in the United Kingdom and with Nos 13, 14, 18, 55, and 114 Squadrons serving in the Middle East and later in Italy. They replaced the Blenheims previously operated by these units. The first raid against the enemy in occupied France took place on February 12, 1942. They took part in attacks on the German warships Scharnhorst, Prinz Eugen, and Gneisenau when they were involved in the famous channel dash during Operation Cerberus.
Many Boston III aircraft were modified to Turbinlite or Intruder configurations. The Boston III (Intruder) aircraft retained their transparent noses but were painted matte black, equipped with flame-damping exhausts, and fitted with a belly gun pack housing four 20-mm Hispano cannon which supplemented the standard Boston III armament. These operated from mid 1942-onwards with Nos 418 (RCAF) and 605 Squadrons. Serials of the DB-7Bs converted to Intruder configuration include W8256, W8262, W8264, W8266, W8268, W8278, W8281, W8283, W8284, W8290, W8292, Z2155, and Z2165. Boston III aircraft fitted with Turbinlites included W8257, W8265, W8275, W8276, and W8300.
W8274, W8277, W8316, W8328, W8341, W8352, W8366, W8369, W8393, W8396, Z2184, Z2214, and Z2270 were converted to Havoc N.F.II. W8401 and Z2189 converted to trainers. Z2200 was transferred to the USAAF.
Engines: Two Wright R-2600-A5B Double Cyclone fourteen-cylinder, twin-row air
cooled radial engines with two-speed supercharger. Rated at 1600 hp
for takeoff and at 1000 feet and 1400 hp at 10,000 feet.
Performance: Maximum speed 311 mph at sea level, 338 mph at 12,500
feet. Cruising speed 273 mph. Initial climb rate 2000 feet per
minute. Service ceiling 27,600 feet. Range with 2000-lb bombload 525
miles. Range with 1000-lb bombload 745 miles.
Weights: 15,051 pounds empty, 20,320 pounds normal maximum takeoff,
23,500 pounds overload.
Dimensions: Wingspan 61 feet 4 inches, length 47 feet 6 inches,
height 17 feet 7 inches, wing area 465 square feet.
Crew: Normal crew complement was four--pilot, bombardier, dorsal
gunner, ventral gunner.
Fuel capacity: 329 Imp gall in four self-sealing tanks in the inner
wing panels, on on each side of the engine nacelles.
Armament: Four fixed 0.303-inch Browning machine guns in nose with
500 rpg. Two 0.303-inch Browning machine guns in flexible dorsal
position. One Vickers K 0.303-inch machine gun in flexible ventral
position. Maximum bombload of 2000 pounds in split bomb bay
comprising four 500-lb bombs. Normal bombload was 1000 pounds
comprising two 500-lb or four 250-lb bombs.