The A-20 series began life as the Douglas Model 7B design, a light bomber attempt originally put to the paper as early as 1936. The United States Army Air Corps (USAAC) showed enough interest after a design review in 1938 that it ordered an operational prototype to be constructed under contract. The first flyable model took to the air on October 26th, 1938, and displayed extremely promising potential for such a design. The aircraft was fast on its twin engines and responsive to the controls with very few negative aspects to her overall design. At any rate, the future of the Model 7B was showing great promise.
With America still in an isolationist mentality despite the worsening situation in Europe (and the world for that matter), the Model 7B was not followed upon by the USAAC and shelved for the time being. Despite this setback, the French and Belgium governments came calling - with some desperation one can imagine - and ordered several hundred Model 7B's for immediate production in February of 1940. These were assigned the official designation of DB-7 and construction covered two distinct production models to become the DB-7A and the DB-7B. An initial batch of 100 DB-7's were constructed and an extended order for 270 more was put into action to help strengthen the ranks. Despite the initiative, only 115 DB-7;s were ultimately delivered to French forces before the collapse of France under German power. Some 95 French-operated DB-7's escaped to North Africa while the remaining models in American hands - and the contract to go along with them - were diverted to British ownership who took over operation of the type as the "Boston". The British Boston series covered three distinct marks as the Boston Mk.I (DB-7), Boston Mk.II (DB-7A) and Boston Mk.III (DB-7B).
RAF Bostons were fielded as day bombers initially, though these met with disastrous results. The type was found to be unsuitable for such a dangerous role and therefore modified into a dedicated night-fighter form. The RAF selected roughly 100 of these Boston light bombers and produced the converted "Havoc", intruder aircraft fitted with the AI Mk IV series of radar in the nose housing and as many as 12 x 7.7mm machine guns to handle the offensive dirty work. Additionally, these converted Bostons were given increased armor protection for the crew and specialized exhaust piping to dampen the flame effects of the engines at night. Essentially, the British RAF gave birth to the "Havoc" series by default, despite its origins as an American airplane. Havocs were first fielded by No. 23 Squadron.
With its newfound weapon, the RAF initiated several interesting - yet costly - projects involving the Havoc. One such initiative involved the "Turbinlite", night-fighting Havoc Mk I models fitted with a 2,700-million candlepower spotlight taking up most of the space in the nose housing. Up to 10 squadrons and 18 months of valuable time and resources went into this project which ultimately proved a failure.
In 1939, the USAAC returned to the DB-7 and re-evaluated its potential for use in the American military. The aircraft was given an extended life now with the initial order of 63 DB-7B platforms. The initial requirement of the USAAC specified a high-altitude capable airframe in the attack bomber role. As such, Douglas produced the design (designated as the A-20) with 2 x turbosupercharged R-2600-7 Wright Cyclone radial engines of 1,700 horsepower each. These initial A-20s were to feature a battery of 4 x 7.62mm (.30 caliber) machine guns in fuselage blister positions. An additional 2 x 7.62mm machine guns would be manned from a dorsal position while a single 7.62mm machine gun was allotted to a manned ventral gun position. Interestingly, rearward-firing 7.62mm machine guns were also introduced in this design, with these being mounted in each engine nacelle. Bombload was a reported 1,600lbs of internal ordnance. Crew personnel amounted to four specialists - a pilot, navigator, bombardier (in a glassed-in nose position) and gunner. Performance specs allowed for a top speed of 385 miles per hour (comparable to fighter performance) and a ceiling of up to 31,500 feet and range totaling some 1,100 miles (ferry range).
After the first A-20 was produced in this fashion, the Air Corps came back with a "modest" change to the requirement, deleting the need for a dedicated high-altitude platform and instead centering on a design capable of handling operations in a low-to-medium altitude zone. As such, the design was revisited and had its turbo-superchargers rightfully removed as they posed no performance benefits at lower operating levels. With the first A-20 completed to the high-altitude specification, the remaining aircraft on order followed this new design direction.
The initial A-20 did go on to serve as the developmental prototype form of the XP-70, the basis for the P-70 dedicated night-fighter. This single A-20 was modified to an extent, having its Wright Cyclone engines changed out for a different model version in the engine series and the problematic turbosuperchargers removed altogether, the latter causing some cooling issues.
143 total contracted A-20A models (based on the DB-7B) were introduced, first as an initial batch of 123 followed up by a smaller batch of 20 more. Differences between the A-20A and the original A-20 model were subtle yet distinct. The most prominent change occurred in the selection of a new Wright R-2600-3/-11 series engines of 1,600 horsepower each (sans the turbosuperchargers required for high-altitude work). First flight of the model was achieved on September 16th, 1940 and saw first deliveries to the 3rd Bomb Group by early 1941. Smaller issue numbers also were allotted to Panamanian and Hawaiian stations as well, extending the types reach somewhat. Armament of A-20A models included the 4 x 7.62mm forward-firing machine guns in fuselage blisters, 2 x 7.62mm machine guns in the dorsal position, 1 x 7.62mm machine gun in the ventral position and the 2 x 7.62mm rear-firing machine guns in fixed engine nacelle positions. Additionally, 1,600lbs of internal ordnance could be carried. The crew remained the initial four personnel as in the base A-20 design. Performance allowed for a top speed of 347 miles per hour, a ceiling of 28,175 feet and an operating range of 1,000 miles.
By October of 1940, the USAAC went on order for more Havocs, these being in the newer A-20B model form based on the DB-7A). The A-20B retained its twin engine light attack bomber role along with the A-model's R-2600-11 series radials and glassed-in nose, framed like that of a greenhouse. A slight variation in the design of this nose glass was the most discerning factor between distinguishing the A-20A and A-20B models. Additionally, armament was lessened to an extent, with the A-20B mounting just 2 x 12.7mm nose-mounted (lower fuselage) machine guns, 1 x 12.7mm machine gun in the dorsal position, 1 x 7.62mm machine gun in the ventral position and the two engine nacelle-mounted rear-firing 7.62mm machine guns. To make up for the lessened offensive firepower, the internal bombload was increased to 2,400lbs. Performance remained comparable with the top speed reported at 350 miles per hour, a range of 2,300 miles and a ceiling of 28,500 feet. The US Navy received eight such aircraft but used them in the target towing role as BD-2's. The Soviet Union became a large operator of the A-20B series under the Lend-Lease program, receiving 665 of the 999 production examples under the agreement.
The A-20C model appeared as an "improved" A-20A model. C-models brought back the cluster of 4 x 7.62mm lower-fuselage nose-mounted armament. 2 x 7.62mm machine guns were appropriated to the dorsal aft gun position along with a 7.62mm machine gun in a flexible ventral tunnel mount. Bombload was an impressive 2,400lbs though this could be supplemented by an internally-held fuel supply instead, increasing the aircrafts range somewhat. Additional improvements over the A-models included self-sealing fuel tanks (almost a prerequisite of any aircraft design going into the Second World War), improved armor plating for improved crew protection and 2 x Wright R-2600-23 Cyclone radial engines of 1,600 horsepower each. A crew of four was still required to operate the light attack bomber. Despite these additions and the aircraft proving heavier than preceding models, the A-20C saw only a slight reduction to overall speed. Performance included a top speed of 342 miles per hour, a range of 2,300 miles and a reduced ceiling of 25,320 feet. 948 examples of the A-20C were produced with these ear-marked for Britain (under designations Boston Mk.III and Boston Mk.IIIa) and Russia under Lend-Lease. However, the American need for such a platform was growing evermore and, as such, the a bulk of these were retained for American use. At least 140 A-20C models were produced by Boeing at their Seattle, Washington plant. Douglas handled the production of their 808 A-20C's at their Santa Monica, California facility.
The A-20D model appeared as a proposed high-altitude variant based on the A-20B. Once again, the idea of a turbosupercharged Wright Cyclone engine was entertained with equal results. The project was cancelled without any production taking place.
The A-20E represented converted A-20A models for use as utility use or developmental test airframes. These models were fitted with the Wright Cyclone R-2600-11 series engines as found on the later batch of A-20A production models (numbering some 20 such aircraft). A total of 17 A-20E converted models existed.
The XA-20F was a single A-20A model converted for use as a weapons test platform. These Havocs had a 37mm T-20-E-1 cannon mounted in a specially-designed nose assembly. General Electric powered turrets replaced the flexible gun mounts of the dorsal and ventral positions, each fitted with 2 x 12.7mm machine guns. Though relegated strictly to testbed use, the remote-controlled turrets were later a feature of the Douglas A-26 Invader series. This aircraft retained the Wright R-2600-3/-11 series of engines in its design.
The A-20G became the largest production run of the Havoc series, numbering some 2,850 total aircraft. The A-20G followed on the heels of the A-20C production model and was a dedicated ground attack platform as opposed to the light bomber designation carried by preceding models. The initial production A-20G block (259 total A-20G-1's) featured the distinctive solid nose assembly mounting 4 x 20mm cannons (deleting the bombardier's nose position and bringing the crew total down to three personnel). The follow-up production block (Block 5) reverted back to a more conventional array of 6 x 12.7mm machine guns as the cannons were prone to jamming and offered up a slow rate of fire. The cannon-armed versions were mostly operated under the Soviet banner via Lend-Lease and understandably proved quite devastating in the ground attack role. Additional armament for either form of this aircraft included 2 x 12.7mm machine guns in a flexible dorsal position and a single 7.62mm machine gun in the ventral position (flexible mount). Bombload totaled 2,000lbs of internally-held ordnance and/or 374-gallon drop tank. Engines for the aircraft were Wright R-2600-23 Cyclone supercharged radials of 1,600 horsepower each. Top speed was 317 miles per hour with a combat range of 950 miles and a modest ceiling of 23,700 feet.
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