The North American Aviation B-25 Mitchell medium bomber played a key role in the Allied victory in World War II around the globe.
Of the 9,800 B-25s built between 1940 and 1945, the U.S. Army Air Forces had more than a couple thousand in service at the height of the bomber’s employment in the war, while Great Britain and the Soviet Union also flew squadrons of Mitchells.
Responding to Air Corps requests for bomber designs, North American created the NA-40 twin engine bomber that first flew in January 1939. The NA-40 had promise, but warranted a redesign that became the B-25 of 1940.
Early B-25s had constant dihedral — the upward slope of the wings to the wingtips. But problems with wallowing that degraded the handling qualities of the first nine B-25s led to wind tunnel testing to correct the issue.
At the University of Washington wind tunnel, as a discouraging day of test runs came to a close, North American decided to saw the outer wing panels off the wooden wind tunnel model and re-attach them with greatly reduced dihedral. The final configuration had the outer wing panels less than a half-degree into negative dihedral. This helped make the B-25 a stable aircraft that served as a bomber, strafer, and later a trainer of airmen.
The small run of initial B-25s was followed by B-25As that had flexible gun mounts, including tail armament, but no power turrets. The B-25B of 1941 introduced a Bendix upper turret and a remotely sighted retractable Bendix belly turret, each packing a pair of .50-caliber machine guns. Optimism over the effectiveness of the two power turrets led to deletion of some hand-held guns, including the early tailgun emplacement.
In B-25s with no tailguns, a clear Plexiglas end cap provided the tail observer’s station. North American described this as “a level platform and a transparent Plexiglas tail cone through which an observer may view bombing results or keep a watch for enemy aircraft.”
The B-25B roared into history on April 18, 1942, when 16 launched from the aircraft carrier USS Hornet to deliver the first strikes on the Japanese homeland in World War II. To discourage tail attacks, they carried dummy wooden gun barrels in the Plexiglas end cap of the fuselage.
The B-25 used two main rivet styles in exterior construction. The forward third of the fuselage and wings and empennage were fabricated with countersunk rivets for reduced drag, while the aft two-thirds of these surfaces used brazier head rivets for an economy of construction where flathead rivets requiring countersunk holes were less necessary.
The Mitchell design hit its stride in production with the B-25C and D-models, similar versions produced at two plants. These B-25s included an autopilot, external bomb racks, and increased gasoline capacity. America really went to war with B-25Cs and Ds.
The XB-25E and XB-25F tested hot air and electric deicing equipment.
The next production variant, the B-25G, was known for its 75 millimeter cannon in a shortened metal nose. Fifty caliber machine guns for ground attack shared space in the nose.
After 405 G-models were delivered, production of a thousand B-25Hs continued the gun nose with a different 75 millimeter weapon plus four blister guns — forward-firing .50-caliber weapons attached to the side of the fuselage that proved useful in strafing.
The exhaust stacks used on the B-25 evolved, with the C-model and later versions incorporating short single stacks with angular shrouds visible around the cowling.
Problems with the upper exhausts cracking over time led to a post-war conversion that kept the lower individual stacks, but routed the exhaust from the top seven stacks into a collector that had one exhaust on the side of the cowling. Many surviving Mitchells have this modification.
Military B-25Cs and later models cruised around 230-233 miles per hour, with a top speed for the C-model listed as 284 and 272 mph for the later J-model.
Production reached its zenith with the B-25J, reverting to a glass bombardier nose and incorporating armament upgrades influenced by combat experience. The H- and J-models moved the top turret forward. The remote lower turret was removed from many C- and D-models, and not used on the later Mitchells.
The introduction of a manned tail gun position in the B-25H and J models resulted in a raised Plexiglas enclosure, sometimes called a doghouse, for the rear gunner. This put the upper portion of the tail gunner’s body in line with the top turret’s two .50-caliber machine guns when they were stowed facing aft.
In operation, the top turret guns had fire interrupters that kept the machine guns from firing into any portion of the B-25’s airframe. But when stowed after firing, it was possible for a single round to cook off from residual gun heat, and a cookoff round could be deadly to the tail gunner.
To protect the rear gunner, two angled steel plates, concealed in streamlined aluminum fairings, protruded from the top of the B-25’s fuselage just aft of the top turret gun muzzles. In the event of a cookoff round, the bullet would strike the angled steel plate and ricochet off in to space, not aft toward the tail gunner.
More than 4,300 B-25Js were built, making the newest Mitchell also the most numerous B-25 model. By the end of the war, many J-models were retained for use as transports and trainers. Hundreds of basic B-25Js acquired later letter designations when they were modified for pilot training and service as trainers for jet fighter radar fire control systems.
B-25s, mostly J-models or modified J-models, also entered the civilian market after the war. They carried executives, hauled freight, served as firefighting air tankers, and filled many roles from the 1940s into the 1960s.
As the need for World War II B-25s diminished, a group of 18 Mitchells was resurrected for the 1970 feature film “Catch-22.”
Derelict former firefighters joined other B-25s brought back to life by movie pilot Frank Tallman. When filming was complete, most of these airworthy B-25s found their way into the growing ranks of the warbird movement or to the many air museums blossoming around the U.S. in the ensuing years.
B-25s remain popular flying warbird attractions at air shows around the United States.