The projected design data for the Model 179 Medium Bomber were accepted by the USAAC on 5 July 1939 and the first Marauder flew on 25 November 1940. The flow of production Marauders began on 25 February 1941 and by the end of 1944 more than 5,150 had been delivered. The Marauder first went into action in the Australian theatre in April 1942.

The B-26 initial production version was powered by two 1,378.6kW Pratt & Whitney R-2800-5 radial engines and carried a defensive armament of five 12.7mm machine-guns in the nose, dorsal turret and tail. Normal bomb load was 907kg but up to 2,631kg could be carried in the tandem bomb bays. The B-26A was similar to the earlier version except for having R-2800-39 engines and minor changes. Similar Marauder I were delivered to the RAF and SAAF in 1942 under Lend-Lease.

The B-26B corresponded to the Lend-Lease Marauder IA and II and was produced in more than one form. Power was provided by R-2800-5 or 1,490.4kW R-2800-41/-43 engines and tail armament was increased to two guns. From B-26B-10 (Marauder II) the wing span was increased from 19.81m to 21.64m; the area of the vertical tail surfaces was also increased; and armament raised to include one fixed and one flexible gun in the nose, four 'package' guns on the sides of the forward fuselage, two guns in the Martin dorsal turret, two flexible waist guns, one ventral-tunnel gun and two tail guns. The front bay could carry two 900kg bombs on special carriers and use of the rear bomb bay was discontinued. The crew was increased from five to seven. The B-26B variants were the most produced of the series.

The B-26C (Marauder II) was the same as the B-26B-10 types but built at the Martin Omaha plant. The single experimental B-26D with exhaust-heated surface de-icing equipment and the single B-26E special stripped model were followed by the B-26F and G (Marauder III). These were similar to the B-26C except for having the incidence of the wings increased by 3 1/2°, no provision for carrying a torpedo, and 11 guns fitted.

Certain examples of earlier B-26 were stripped of armament and adapted for training and general utility duties, particularly high-speed target-towing. These were originally known as AT-23 but were subsequently redesignated TB-26. A number of TB-26G were also built. The designations JM-1 and JM-2 applied to stripped versions of the B-26C and B-26G respectively, used by the US Navy for target-towing and other general utility duties. The JM-1P was equipped for photographic reconnaissance.