In 1940, American military planners recognized that the war in Europe would create such an increased need for military aircraft that existing manufacturing facilities would be unable to keep up with the demand. Consequently, the US government approved the construction of of a series of entirely new aircraft plants. These new plants would be constructed mainly in the Midwest, well isolated, it was hoped, from the threat of bombing raids from either German or Japanese aircraft. Ownership of these plants would be retained by the government, but they would be leased to private aircraft manufacturers for the purpose of fulfilling military contracts.
An initial order for 1200 B-25D (NA-82) bombers was approved on June 28, 1941. North American Inglewood manufactured and supplied the parts for the first 100 B-25Ds built at Kansas City. The first two B-25Ds were accepted in February of 1942. Subsequent B-25Ds were built almost entirely by Fisher and by North American/Kansas. Beginning with B-25D serial number 41-29748, Fisher supplied outer wings, fuselage side panels, control surfaces, and transparent enclosures to Kansas City for mating with center sections and with other parts manufactured there.
The B-25D was virtually identical to the B-25C, and many of the innovations introduced on the B-25D production line at Kansas City paralleled those introduced on the B-25C line at Inglewood. Generally, it was impossible to tell the difference between a B-25C and a B-25D without a knowledge of the serial numbers.
Beginning with the B-25D-1 production block, external wing bomb racks were provided, additional self-sealing tanks were installed in the outer wing panels (adding 304 US gallons to the fuel capacity), carburetor air filters were installed, self-sealing oil tanks were provided, provisions were made for the installation of torpedo racks, a scanning blister was installed over the navigator's compartment, Bendix Amplidyne turrets were installed, and the flame-dampening "finger"-type engine exhausts were installed. The external bomb racks could carry six to eight bombs in the 100-325 pound range.
In the B-25D-5 production block, the 0.30-inch machine gun in the nose was replaced with a pair of fixed 0.50-inch and a single flexible 0.50-inch machine guns. This made them different from the B-25C's, which had only a single 0.50-inch fixed gun in the nose. An improved scanning lens for the sighting system for the retractable ventral turret was provided, and provisions for a 585-gallon droppable bomb-bay fuel tank were installed on every third airplane through 41-30532. Provisions for additional cabin heating were added on B-25D-5 No. 41-30057 onward.
On the B-25D-10 production block, additional provisions were made for better winterization, a remote reading compass was installed, emergency hydraulic landing gear lowering mechanisms were provided, and the conduit shielding box was eliminated.
Effective on B-25D-15 and subsequent production blocks, the flame-dampening "finger"-type stack exhaust collector was replaced with individual Clayton "S"- shaped exhaust stacks connected to each cylinder, with individual cutouts and fairings being provided in the cowling where the stacks protruded.
The North American B-25 was among the famous twin engine medium bombers used during World War II. It was the most widely produced American twin engine combat aircraft, with approximately 10,000 produced, in a total of 8 major models. No doubt, part of its heroic stature derives from its namesake, the outspoken Gen. Billy Mitchell who proved once and for all that bombers could destroy targets, and that wars would nevermore be decided only on land or sea. The B-25 achieved worldwide fame on April 18, 1942. Sixteen B-25's, under the command of Lt. Col. James Doolittle, were launched from the aircraft carrier Hornet in a daring raid on five Japanese cities including Tokyo, Osaka, Yokohama, Nagoya, and Kobe.
B-25 Mitchells fought in every theatre of the Second World War and operated in many roles including tactical bombing, low-level strafing and skip bombing and anti-shipping strikes. In addition to service with the U.S. Army Air Force, these bombers were also used to good effect by the U.S. Navy and Marine Corps and the air forces of 17 foreign countries. The last operational B-25 was finally retired from the U.S. Air Force inventory in January 1959.