The Douglas XA-26 prototype (41-19504) first flew on 10 July 1942 at Mines Field, El Segundo, with test pilot Benny Howard at the controls. Flight tests revealed excellent performance and handling, but there were problems with engine cooling which led to cowling changes and omission of the propeller spinners on production aircraft, plus modification of the nose landing gear after repeated collapses during testing.

The A-26 was originally built in two different configurations. The A-26B had a "solid" nose, which originally could be equipped with a combination of anything from .50 caliber machine guns, 37mm auto cannon, 20mm or even a 75mm pack howitzer, but normally the solid nose version housed six (or later eight) .50 caliber machine guns, officially termed the "all-purpose nose", later commonly known as the "six-gun nose" or "eight-gun nose". The A-26C's "glass" nose, officially termed the "Bombardier nose", contained a Norden bombsight for medium altitude precision bombing. The A-26C nose section included two fixed M-2 guns, later replaced by underwing gun packs or internal guns in the wings.

After about 1,570 production aircraft, three guns were installed in each wing, coinciding with the introduction of the "eight-gun nose" for A-26Bs, giving some configurations as many as 14 .50 in (12.7 mm) machine guns in a fixed forward mount. An A-26C nose section could be exchanged for an A-26B nose section, or vice versa, in a few man-hours, thus physically (and officially) changing the designation and operational role. The "flat-topped" canopy was changed in late 1944 after about 820 production aircraft, to a clamshell style with greatly improved visibility.

Alongside the pilot in an A-26B, a crew member typically served as navigator and gun loader for the pilot-operated nose guns. In an A-26C, that crew member served as navigator and bombardier, and relocated to the nose section for the bombing phase of an operation. A small number of A-26Cs were fitted with dual flight controls, some parts of which could be disabled in flight to allow limited access to the nose section. A tractor-style "jump seat" was located behind the "navigator's seat." In most missions, a third crew member in the rear gunner's compartment operated the remotely-controlled dorsal and ventral gun turrets, with access to and from the cockpit only possible via the bomb bay when that was empty.