Science Fair Project Encyclopedia
|Douglas DB-7B Boston III|
|First flight||October 26, 1938 (Model 7B)|
|Length||47 ft 4 in||14.4 m|
|Wingspan||61 ft 4 in||18.7 m|
|Height||18 ft 1 in||5.5 m|
|Wing area||464 ft²||43.1 m²|
|Empty||0 lb||0 kg|
|Loaded||24,000 lb||10,880 kg|
|Engine||2 × Wright GR-2600-A5B Cyclone|
|Power (each)||1,350 hp||1,010 kW|
|Maximum speed||350 mph @ 12,000 ft||563 km/h @ 3,660 m|
|Combat range||1,080 miles||1,740 km|
|Service ceiling||23,600 ft||7,190 m|
|Rate of climb||ft/min||m/min|
|Guns||8 × 0.303 in (7.7 mm) machine-guns|
|Bombs||2,000 lb||900 kg|
Douglas Model 7A
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 (340 kW) Pratt & Whitney R-985 Wasp Junior engines mounted on a high-mounted wing. It was estimated it could have carried 1000 lb (450 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 not progressed.
Douglas Model 7B
In the autumn of the same year, the Army Air Corps issued its own specification for an attack aircraft. The Douglas team, now headed by Heinemann, took the model 7A design, upgraded massively to 1100 hp (820 kW) Pratt & Whitney R-1830 S3C3-G Twin Wasp engines, and submitted the design as the Model 7B. It faced competition from the North American NA-40, the Stearman X-100 , and the Martin 167F . The Model 7B was maneuverable and fast, but did not attract any US orders.
The model did, however, attract the attention of a French Purchasing Commission visiting the USA. The French participated in the flight trials, discretely so as not to attract criticism from US isolationists. The French ordered 100 production aircraft, which order was increased to 270 when the war began.
The French order called for substantial modifications, and the new designation DB-7 (for Douglas Bomber 7) was introduced. It had a narrower deeper fuselage, 1000 hp (750 kW) Pratt & Whitney R-1830-SC3-G radials, and French guns and metric instruments. Midway through the delivery, engines were switched to 1100 hp (820 kW) Pratt & Whitney R-1830-S3C4-G. The French designation was DB-7 B-3 (the B-3 meaning "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 invaded France on May 10, 1940 the 64 available DB-7s were deployed against the advancing Panzers. Before the armistice they returned to North Africa, and some were used by Vichy French forces on the Axis side. A few were still operating under Free French colours to the end of the war in Europe.
Boston Mks.I & II
The Royal Air Force agreed to take up the balance of the now-frustrated French order was diverted to Britain, and the aircraft were designated Boston Mk I or Mk II according to the earlier or later engine type.
Havoc Mk I
The aircraft was not really suitable for RAF use as its range was too limited for day-time raids on Germany. Many of the Boston Mk II, plus some re-engined Mk Is, were converted for night time duties - either as intruders with 2,400 lb (1,100 kg) bombs, or as night-fighters with AI Mk IV radar. These were known collectively as Havoc Mk I. 181 Bostons were converted to Havocs. Havoc Intruders caused considerable damage to German targets.
20 Havocs were Havoc Mk I (Pandora) intruder aircraft, trailing the Long Aerial Mine (LAM) which was an explosive charge trailed on a long cable in the path of enemy aircraft in the hope of scoring a hit. It was not successful and they were converted back to Mk I intruders.
A further 31 were Havoc Mk I Turbinlite fitted with a 2700 million candela (2.7 Gcd) searchlight in the nose. They were unarmed and were supposed to illuminate targets for accompanying Hurricane fighters, but in practice they just made nice bright targets for German gunners.
DB-7A Havoc Mk II
The French Purchasing Commission ordered a further 200 bombers, to be fitted with 1600 hp (1190 kW) Wright R-2600-A5B Double Cyclone engines. This variant was designated DB-7A by Douglas. None had been delivered before the fall of France, and they served instead as night-fighters with the RAF under the name of Havoc Mk II. They had an impressive top speed of 344 mph (550 km/h) at altitude. 39 of them were used briefly in Turbinlite roles.
DB-7B Boston Mk III, DB-73, DB-7C
The DB-7B was the first batch of the series to be actually ordered by Britain, in February 1940. Powered by the same engines as the DB-7A, with better armor and, crucially, larger fuel tanks, these were at last suitable for British use in the light bomber role. This was the batch for which the name Boston was first reserved, but since the commandeered DB-7s entered service first, this batch became known as Boston Mk III. Amongst other operations they took part in the attacks on the Scharnhorst, Gneisenau and Prinz Eugen during their escape dash through the English Channel on February 12, 1942. 300 were delivered and some were used in Intruder and Turbinlite roles.
The DB-73 was a very similar design for France, which again were diverted to England as Boston Mk IIIA. Many of these were built under licence by Boeing. Events further overtook this shipment after the German attack on the Soviet Union on June 22, 1941 and the Attack on Pearl Harbor on December 7, when many Bostons were sent to Russia and many more retained by the USAAF for its own use. Some 22 were also sent to the RAAF.
The DB-7C was a Netherlands order intended for service in the Netherlands East Indies, but the Japanese invasion was complete before they were delivered and they were sent instead to the Soviet Union under Lend-Lease.
The original American indifference to the Model 7B was overcome by the improvements made for the French and British, and the Army Air Corps ordered two models, the A-20 for high-altitude bombing and the A-20A for lower-altitude work. 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 un-supercharged R-2600-11, 59 as P-70 fighters and 3 as F-3 reconnaissance panes (of which more later).
The US Army ordered 123 A-20As with R-2600-3 engines, and a further 20 with more powerful R-2600-11. They entered service in spring, 1941. The Army liked the A-20A because of its excellent performance and because it had no adverse handling characteristics. 9 of them were transferred to Australia in 1943. The British name Havoc was adopted for the A-20A.
The A-20B received the first really large order - 999 - from the US Army. They resembled the DB-7A rather than the DB-7B, with light armor and stepped rather than slanted glazing in the nose. In fact, 665 were exported to the Soviets, so rather few actually served with the USAAC.
The A-20C was an attempt to standardize on a common British and American version, produced from 1941. It reverted to the slanting nose-glass and had RF-2600-23 engines, self-sealing fuel tanks and additional armor. They were equipped to carry an external 2000 lb (900 kg) naval torpedo. 948 were built for Britain and the Soviet Union, but many were retained by the USAAF after Pearl Harbor.
A-20G & A-20H
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 contain four 20 mm cannon and two 0.50 in (12.7 mm) Colt-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 1600 hp (1200 kW) R-2600-23. US A-20Gs were used on low-level sorties in the New Guinea theatre.
The A-20H was the same continued with the 1700 hp (1270 kW) R-2600-29. 412 of these were built. The take-off weight was raised to 24,170 lb (10,960 kg).
A-20J, Boston Mk IV
The A-20J carried an additional bombardier in an extended plexiglass nose section. These were intended to lead bombing formations, with the following standard A-20s dropping their bombs when signalled by the leader. 450 were built, 169 for the RAF which designated them Boston Mk IV from summer 1944.
A-20K, Boston Mk 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, so most of the production run of A-20s were converted to P-70 night-fighters. They were equipped with British AI Mk IV radar, the glazed nose often painted black to reduce glare, and had four 20 mm cannon. Further P-70 variants were produced from A-20C G and J variants, later models having American centimetric radar fitted.
The F-3A was a conversion of 46 A-20J and K models for night-time photographic reconnaissance (F-3 were a few conversions of the original A-20).
|Related development||Douglas DC-5|
A-17 - A-18 - A-19 - A-20 - A-21 - A-22 - A-23
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