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Updated: Mar 23, 2020

As a result of developing global threats, United States aircraft manufacturers were directed in 1938 to develop a new class of medium bomber, with a greater speed, range and load than the B-18 light bombers of the day, but smaller than the Boeing B-17 heavy bomber. This new medium bomber would go on to become the most successful aircraft of its type-the North American B-25 Mitchell. During this blog, we'll trace both the development of the Mitchell as well as its distinguished combat career.

Back as far as 1936 North American Aviation had developed a medium bomber design for USAAC evaluation, designated the XB-21. This aircraft was a twin engine type with each nacelle fitted to each wing mainplane unit outboard of the centralized fuselage. The cockpit was stepped with the nose glazed for a navigator/bombardier's position with a crew of eight. The tail unit was composed of a single vertical fin with low-set horizontal planes. The XB-21 used a tail-dragger landing gear arrangement and was powered by two Pratt & Whitney R-1280-A Twin Hornet turbosupercharged, air cooled piston radial engines with an internal bomb load of 10,000 lbs. At $122,000 per copy, the XB-21 was an expensive plane for its day. Though the Air Corps decided upon the Douglas B-18 after a fly off of the two aircraft in December 1936, North American engineers gained valuable experience from the XB-21 project.

The XB-21 (now NA-21) was North American's first twin engine product and the company was determined to produce a more refined, state of the art aircraft. With development entering a rapid pace, the NA-21 was now designated the NA-40 with the first test flight scheduled for January 1939. However, the NA-40 was fitted with more powerful engines and additional improvements in March 1939. Tragically, the test flight of the prototype aircraft ended in a crash the following month. Not to be discouraged, the North American design team proceeded with a number of modifications to the NA-40, now designated the NA-62. The NA-62 design was based on a new series of USAAC requirements, for a medium bomber type capable of speeds near 300 mph with a range of 1,200 miles and a payload of 2,400 lbs. After a successful test flight in September 1939, the Air Corps accepted the NA-62 prototype with the aircraft entering production as the B-25. Due to the war in Europe in full swing, production of the B-25 accelerated with the addition of such features as the addition of anhedrals to the outer wing panels, giving the wings a gull shape, as well as an increased surface area to both tail fin panels, both measures designed to provide flight stability. Powered by two Wright R-2600-9 radial piston engines of 1,350 hp each, with a bomb load of 3,600 lbs. Though its defensive armament consisted of only three machine guns, one .30 caliber in the nose, another in the waist, with a lone .50 caliber gun in the tail, this problem would be remedied in the following model, the B-25A, which offered additional .50 caliber machine guns, as well as defensive improvements, such as self-sealing fuel tanks and an enhanced tail gunner station.

When the United States entered World War II in 1941 only one B-25 group was operational, and it was temporarily assigned to fly anti-submarine patrols off the east coast. The first B-25s stationed overseas operated from Egyptian bases, flying missions against Axis airfields and mechanized supply columns. Mitchells later provided ground support in Sicily and the Italian mainland and was effective in strikes against both road and rail links. These groups were the only USAAF B-25 units to operate in the European theater. In early 1943, the RAF began to update its bomber inventory, replacing older Douglas Bostons and Vickers Wellingtons with the Mitchells. These aircraft initially performed bombing raids over continental Europe, acting as supply aircraft following the Normandy invasion. A number of B-25s were also supplied to both the Royal Canadian and Soviet air forces through the Lend-Lease program .

The B-25 had the distinction of being the only medium bomber to operate from an aircraft carrier during World War II. Lieutenant Colonel Jimmy Doolittle, who led the raid was in need of an aircraft capable of taking off from an aircraft carrier (500 ft.) with the required bomb and fuel load. To select the desired plane, the Mitchell was pitted against another medium bomber, the Martin B-26 Marauder. The B-25 proved a better aircraft for carrier operations, as the result of a fly off between the two and the Mitchell was modified accordingly. All 16 B-25s took off from the USS Hornet in April 1942, bombing Tokyo and four other cities without loss, though only one plane landed intact in Siberia with eleven crew members lost of the original eighty. There were plans to operate Mitchells from carriers later in the war, but these never materialized.

The Mitchell made perhaps its greatest impact in the Southwest Pacific area. It was from operations in this area, that the B-25's potential as a ground attack plane developed. While the Mitchell was designed for bombing from medium altitudes, the jungle environment of the Pacific made such tactics impractical, dictating low-level approaches to targets. Another tactic tailored to the B-25 was skip bombing, in which the attacking bomber would fly just above sea level, dropping its bombs on the water at a predetermined point, bouncing into the side of an enemy ship. On land, the Mitchells made low-level runs on Japanese airfields and ground installations with the use of parafrag bombs. Such bombs allowed the B-25s to approach the target at low altitude, yet avoid their blast with the parachutes delaying ground contact until the aircraft were clear of the area. With the fragmentation feature, they were effective against both personnel and parked aircraft. While the Mitchell went through a number of modifications during the war, the most important was the gunship, or strafer version. When Maj. Gen. George Kenney took command of the Fifth Air Force in August 1942, he met a Third Attack Group maintenance officer, Major Paul Gunn, who was in the process of converting an A-20 attack bomber into a strafer plane. Gunn's conversion involved removing the bombardier's station and replacing it with four .50-caliber machine guns, with two others mounted on the sides of the fuselage. General Kenney was so impressed with Gunn's effort, that he ordered a number of both A-20s and B-25s converted as strafers. The converted B-25s had a solid nose, like the A-20s, but were armed with eight .50-caliber machine guns with two more mounted on each side of the fuselage, which proved effective for both shipping and ground attacks. Some strafers mounted a 75 mm cannon in the nose for anti-shipping missions. Though it offered a lot of firepower, the 75 mm gun had a tendency to stop the plane in mid air.

The B-25 was the most produced medium bomber of World War II. It served in all areas of the conflict from beginning to the end. The Mitchell was a versatile aircraft, as evidenced by its success in the Southwest Pacific, destroying both Japanese ships and aircraft in large numbers. Though the USAAF staff considered replacing it with the newer A-26 invader, which had a number of flaws, General Kenney wouldn't trade his Mitchells for any other medium bomber.

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