With design beginning as early as 1940, the Invader was first flown on July 10th, 1942 as the XA-26 pre-production prototype. The XA-26 appeared as a successor to the Douglas A-20 Havoc, an aircraft of similar role and design layout and featured a glass nose and 5,000 of internal and external ordnance capability. Armament consisted of 2 x 12.7mm forward-mounted machine guns and a remote-control periscope-fired dorsal and ventral barbette, each with 2 x 12.7mm machine guns (an arrangement very similar to the production A-26C models). A mockup was completed and showcased in early 1941 with the contract finalized in June of that year. The contract originally called for just two prototype aircraft types that included the XA-26 light attack bomber and the XA-26A dedicated night attack fighter.
The resulting tests revealed some structural issues with the nose landing gear - it proving prone to collapse - and, as such, the component was redesigned. Other modifications centered on engine overheating to which the original propeller spinners (intended to promote streamlining of the aircraft) were removed for improved cooling airflow and the engine cowlings were redesigned for better performance. Overall, the tests proved the aircraft design sound and capable of great speed. Handling was regarded as above average and very responsive. What made the XA-26 unique was its single pilot cockpit, not requiring the need for a dedicated co-pilot and thusly keeping the fuselage a slender shape ala the Northrop P-61 Black Widow and Douglas A-20 Havoc. The XA-26 was ordered by the USAAF with the series designation of A-26 and consisted of several major variants, though no A-26A production model existed. Production of the A-26 series progressed slowly as most of Douglas' plants were tied to previous contract aircraft production. As such, the A-26 would have to wait until 1944 to see any complete forms.
The XA-26A model was a prototype night-fighter and attack platform. This model deserves mention for its dedicated role introducing the solid nose covering the search radar system. A ventral gun tub was devised to compensate for the aircrafts lack of forward armament and resulted in a battery of 4 x 20mm cannon. Bombload was a diminutive 2,000lbs thanks to the space committed to the radar system and cannon armament (and ammunition). The dorsal remote-controlled barbette was retained with its 2 x 12.7mm machine guns. The Northrop P-61 Black Widow beat the XA-26A to the night-fighter punch, being already in production and offering the same performance specifications as the XA-26A. Power was to be from a pair of Pratt & Whitney R-2800-27 radial engines rated at 2,000 horsepower each. The XA-26A existed in just a single prototype offering.
In June of 1942, the initial Army Air Corps contract was amended to and added a third prototype aircraft in the form of a single XA-26B example completed at the Douglas El Segundo plant. This aircraft model was to fit the mold of low-altitude attacker platform with Pratt & Whitney R-2800-27 engines of 2,000 horsepower each and an internal/external bomb capacity of 6,000lbs with a crew of three. Visually, the XA-26B was similar to the XA-26A model series and featured a similar solid nose covering but this over an embedded 75mm cannon. In theory, the aircraft proved a sound weapon but in practice, the cannon was slow to fire and prone to jamming with a high level of required maintenance. This forced the Douglas team to try a host of alternative armament combinations including the use of 37mm cannon or heavy machine guns or both. Some early production B-models were pulled off the line for testing various armament load outs. Impressive ideas were covered but proved fleeting such as mounting the 75mm cannon with two 12.7mm machine guns or twin 37mm cannons with 4 x 12.7mm machine guns. This developmental delay eventually resulted in an early batch of production B-models fitted with 6 x 12.7mm machine guns and a later block of production B-models with 8 x 12.7mm machine guns as production of the type had already begun while testing of the armament ensued. The USAAF accepted the design on June 30th, 1943.
The A-26B model series became the production version of the XA-26B, with its mounted collection of heavy machine guns in a solid nose assembly and a top speed of 350-355 miles per hour with her Pratt & Whitney R-2800-27, -71 or -79 series engines of 2,000 horsepower. A crew of three operated this type and consisted of a pilot, navigator/loader and gunner. Some 1,355 B-26B model series aircraft were eventually produced along with 25 other aircraft that were never delivered. Production was handled at two Douglas plants - one in Long Beach, California and the other in Tulsa, Oklahoma.
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