This aircraft was one of the most controversial American combat aircraft of the Second World War. It was primarily used in Europe, and was in fact numerically the most important USAAF medium bomber used in that theatre of action. However, on four occasions, investigation boards had met to decide if the development and production of this plane should continue. It survived all attempts to remove it from service, and by 1944, the US 9th Air Force had the lowest loss rate on operational missions of any American aircraft in the European theatre, reaching a point less than one half of one percent.
 

A total of 5157 planes were built. Although on paper it was a more advanced aircraft than its stablemate, the North American B-25 Mitchell, it was built in much fewer numbers because it was more expensive to manufacture and had a higher accident rate.

One of the most commonly-asked questions is the difference between this plane and the Douglas B-26 Invader. They were two completely different aircraft and had been designed to completely different requirements. The Douglas B-26 Invader had been originally been designated A-26, and was a twin-engined attack bomber intended as a successor to the Douglas A-20 Havoc. In 1948, the newly-independent Air Force decided to eliminate the A-for-Attack series letter as a separate designation, and the A-26 Invader was redesignated B-26, in the bomber series. There was no danger of confusion with this plane, since it was by that time no longer in service with the US Air Force.

The history of this airplane dates back to early 1939. Both the North American B-25 Mitchell and this plane owe their origin to the same Army Air Corps specification. On March 11, 1939, the Air Corps issued Proposal No. 39-640 for the design of a new medium bomber. According to the requirements listed in the specification, a bombload of 3000 pounds was to be carried over a range of 2000 miles at a top speed of over 300 mph and at a service ceiling exceeding 20,000 feet. The crew was to be five and armament was to consist of four 0.30-inch machine guns. The proposal called for either the Pratt & Whitney R-2800, the Wright R-2600, or the Wright R-3350 radial engine.

Requests for proposals were widely circulated throughout the industry. Proposals were received from four manufacturers. The proposal of the ultimate manufacturer was a company of Middle River, Maryland (near Baltimore). The plane was assigned the company designation of Model 179. The manufacturer assigned 26-year old aeronautical engineer Peyton M. Magruder as Project Engineer for the Model 179. Magruder and his team chose a low-drag profile fuselage with a circular cross section. Since the Army wanted a high maximum speed but hadn't specified any limitation on landing speed, the team selected a high-mounted wing with a wingspan of only 65 feet. Its small area gave a wing loading of more than 50 pounds per square foot. The wing was shoulder-mounted to leave the central fuselage free for bomb stowage. The wings were unusual in possessing no fillets. The engines were to be a pair of 1850 hp Pratt & Whitney R-2800-5 Double Wasp air-cooled radials, which were the most powerful engines available at the time. Two-speed mechanical superchargers were installed in order to maintain engine power up to medium altitudes, and ejector exhausts vented on each side of the closely-cowled nacelles. The engines drove four-bladed 13 foot 6 inch Curtiss Electric propellers. Large spinners were fitted to the propellers, and root cuffs were added to aid in engine cooling.

The armament included a flexible 0.30-inch machine gun installed in the tip of a transparent nose cone and operated by the bombardier. Two 0.50-inch machine guns were installed in a dorsal turret located behind the bomb bay just ahead of the tail. This was the first power-operated turret to be fitted to an American bomber. Another 0.30-inch flexible machine gun was installed in a manually-operated tunnel position cut into the lower rear fuselage. There was a 0.50-inch manually-operated machine gun installed in a pointed tailcone. The tail gunner had enough room to sit in an upright position, unlike the prone position that had been provided in the early B-25.

There were two bomb bays, fore and aft. The bomb bay doors were unusual in being split in tandem, the forward pair folding in half when opened and the aft set being hinged normally to open outward. Two 2000-lb bombs could be carried in the main bomb bay, but up to 4800 pounds of smaller bombs could be carried if the aft bay was used as well.

Detailed design of the Model 179 was completed by June of 1939. On July 5, 1939, the Model 179 was submitted to a Wright Field Board. This design was rated the highest of those submitted, and on August 10, 1939, the Army issued a contract for 201 Model 179s. This contract was finally approved on September 10. At the same time, the competing North American NA-62 was issued a contract for 184 examples under the designation B-25. Since the design had been ordered "off the drawing board", there was no XB-26 as such.

Although the first plane had yet to fly, orders for 139 planes with self-sealing tanks and armor were issued on September 16. Further orders for 719 planes on September 28, 1940 brought the total order to 1131 aircraft.

Early wind tunnel test models of this aircraft had featured a twin tail, which designers thought would provide better aerodynamic control. This was dropped in favor of a single fin and rudder so that the tail gunner would have a better field of view.

The plane had a semi-monocoque aluminum alloy fuselage fabricated in three sections. The fuselage had four main longerons, transverse circular frames, and longitudinal stringers covered by a metal skin. The mid section with the bomb bays was built integrally with the wing section. The retractable tricycle landing gear was hydraulically actuated. The nosewheel pivoted 90 degrees to retract into the nose section, and the mainwheels folded backwards into the engine nacelles. The tail fins were of smooth stressed skin cantilever structure. The elevators were covered with metal skin, but the rudder was fabric covered.

The first plane (c/n 1226, USAAF serial 40-1361) took off on its maiden flight on November 25, 1940, with chief engineer and test pilot William K. Ebel at the controls. It initially flew without any armament fitted.

The first 113 hours of flight testing went fairly well, and there were few modifications needed. However, a slight rudder overbalance required that the direction of travel of the trim tabs be reversed.

Since there was no prototype, the first few production aircraft were used for test purposes. On February 22, 1941, the first four planes were accepted by the USAAF. The first to use the plane was the 22nd Bombardment Group (Medium) based at Langley Field, Virginia, which had previously operated Douglas B-18s.

A series of failures of the front wheel strut resulted in a delay in bringing the airplane to full operational status. Although the forward landing gear strut was strengthened in an attempt to correct this problem, the true cause was an improper weight distribution. The manufacturer had been forced to deliver the first few planes without guns, and had trimmed them for delivery flights by carefully loading service tools and spare parts as ballast. When the Army took the planes over, they removed the ballast without replacement and the resultant forward movement of the center of gravity had multiplied the loads on the nosewheel, causing the accidents. The installations of the guns corrected the problem.

The last aircraft was delivered in October of 1941. That month, the  Middle River production line shifted over to the "A" version.