North American B-25 Mitchell
Twin-Engine Medium Bomber Aircraft
The North American B-25 Mitchell proved a crucial medium bomber component to the Allied war effort during World War 2.
Authored By: Staff Writer | Last Edited:
The United States Army Air Corps (USAAC) fielded two major medium bomber types during World War 2 (1939-1945) - the Martin B-26 "Marauder" and the North American B-25 "Mitchell". Both were designed during the same pre-war period with the former's production totaling 5,288 and the latter's registering 9,816 before the end. The Mitchell's legacy was solidified by its use in the 1942 "Doolittle Raids" which brought the war to Japanese soil for the first time. The medium bomber went on to become one of the classic American aircraft of the war and fulfilled its various over-battlefield roles faithfully.
The Need Grows
By the late 1930s, with emerging threats in Japan, Italy and Germany, it was pressed upon American aircraft manufacturers to deliver on a new generation of fighters, bombers and attack platforms. In a March 1938 release by the USAAC, a specification was put forth calling for a design capable of reaching speeds in excess of 200 miles-per-hour out to a range of 1,200 miles with a bomb load of up to 1,200lb. Lofty goals for the period to-be-sure but the need was becoming desperate to better help the United States military (and its allies) contend with new aircraft developments being witnessed overseas.
Back in 1936, North American Aviation (NAA) had developed a medium-class bomber for evaluation by the USAAC as the "XB-21". This entry was a twin-engine type with each nacelle fitted to each wing mainplane member outboard of the centralized fuselage. The cockpit was stepped with the nose glazed for a navigator/bombardier's position and the crew complement numbered up to eight personnel. The tail unit incorporated a single vertical fin with low-set horizontal planes. For ground-running, a tail-dragger landing gear arrangement was used. Power was from 2 x Pratt & Whitney R-1280-A "Twin Hornet" turbosupercharged air-cooled radial piston engines. Defensive armament was centered on 5 x 0.30 caliber M1919 air-cooled machine guns while the bomb load could reach up to 10,000lb in the internal bay.
Only one prototype of the XB-21 was completed to the tune of $122,000 USD and flown, this for the first time on December 22nd, 1936. The XB-21 competed directly with another twin-engine design of the period that would eventually be adopted by the USAAC - the Douglas B-18 "Bolo" (detailed elsewhere on this site). The USAAC still held some interest in the XB-21 for they contracted for multiple evaluation models but, in the end, only a single form was ever completed. The Bolo went on to bigger and better things in the pre-war world, production reaching 350 units, leaving the XB-21 without a role or interested customer.
The NA-21, being North America's first twin-engine product, provided company engineers with priceless experience in designing, developing and selling a combat warplane to the United States military. The framework was more or less set up for the company to deliver a more modern, thoroughly-refined aircraft in the coming years and this new initiative (the "NA-40") moved at such a pace that a first-flight of a revised form was had as soon as late-January 1939. Before the end of March 1939, the aircraft was fitted with more powerful engines to extract performance gains and other facets of its design were ironed out for the better. That same month, the now-NA-40B was readied for evaluation by the USAAC and faced competition from designs offered by Douglas, Martin, and Stearman. The NA-40B failed to secure its future in the coming weeks and, on April 11th, 1939, the prototype was doomed in a crash.
The NA-62 Becomes the B-25
Undeterred, North American Aviation continued to pressed on and began molding the already-completed work of the NA-40B into the new "NA-62". The NA-62 was fleshed out to meet a newer USAAC requirement for an all-modern medium bomber type with speeds nearing 300 miles-per-hour out to a range of 1,200 miles with a 2,400lb war load. In September of 1939, USAAC authorities liked what they saw and committed to the NA-62 - the war in Europe (World War 2) had just broken out on September 1st, 1939 so there was a sense of urgency now. The NA-62 would enter USAAC service under the "B-25" designation and to be fielded side-by-side with a competing medium bomber design, the Martin B-26 "Marauder" (detailed elsewhere on this site).
Production of the new North American bomber ramped up and, following the ninth completed example, the company addressed stability of its product by adding anhedral to the outer wing panels (that is those panels outboard of the engine nacelles). The vertical tail fins also had their surface area increased to add to stability and control.
Early Mitchell Marks
Initial production forms were designated simply as "B-25" and carried 2 x Wright R-2600-9 radial piston engines of 1,350 horsepower each. The bomb bay could accommodate up to 3,600lb of droppable stores and defense was through just three 0.30 caliber machine guns - one fitted at the nose, one at the waist (beam) position, and the final installation in a ventral mounting. A single 0.50 caliber heavy machine gun was installed at the tail to better protect the aircraft's more vulnerable rear. Production ended after 24 examples and the fleet more-or-less served as pre-series aircraft pending the arrival of the B-25A models.
The B-25A was the first-in-line to be deemed combat-capable. To the base form was added better survivability features such as self-sealing fuel tanks and a revised tail-gunner's station. However, this mark only saw production reach 40 total units before attention had shifted to the upcoming B-25B.
The B-25B was improved by way of twin-gunned (2 x 0.50 cal) dorsal turret and remote-controlled, retractable ventral turret. The B-model was the first definitive mark in the series with production reaching 120 units. Some were supplied to the British Royal Air Force via Lend-Lease and served locally under the designation of "Mitchell Mk.I".
The Doolittle Raids of 1942
B-25B aircraft were selected for the famous Doolittle Raid in April of 1942 which showed the Japanese that their homeland could be reached by the United States. Sixteen B-models were used in this daring mission which occurred just four months after the Japanese attack on Pearl Harbor. The bombers were launched from USS Hornet. Of the 80 airmen involved, 69 made the eventual trip home - though fifteen of the bombers crash-landed en route to China.
Mitchell Models Continued...
The B-25C was brought online as an improved form of the preceding B-25B mark. The engines were now switched to 2 x Wright R-2600-13 series air-cooled radial piston engines with added much needed power. The nose section was upgraded by the addition of 2 x 0.50 cal HMGs with one being trainable and the other fixed to protect against head-on attacks. The navigator was given a sighting blister to better account for the bomber's position when en route and anti-icing equipment was installed for cold weather service. The C-model quickly leaped out in front of all other Mitchell bombers for 1,625 total aircraft were built to the standard. Its reach was such that the British, Canadians, Chinese, and Dutch all became recipients of this much-needed bombing platform. For the British, the new model was known as the "Mitchell Mk.II".
The B-25D was a similar mark to the C-model but its production was handled in Kansas City, Kansas (as opposed to Inglewood, California for all others prior). D-model aircraft were also fashioned to photographci-reconnaissance platforms by incorporation of photography gear (3 x K.17 cameras) and operated as the "F-10". In 1944, at least four D-models were further converted to serve in the weather reconnaissance role.
The XB-25E was a single B-25C set aside to be used as a test bed for more advanced anti-icing/de-icing equipment. The XB-25F-A was similarly used. The XB-25G was a single Mitchell modified for the gunship role. Its nose assembly was shrouded over and carried 2 x 0.50 cal HMGs along with a single 75mm M4 automatic cannon for ground-attacking.
The XB-25G was successfully tested and led to the development of the B-25G. Four-hundred production models were completed to this standard. In service, these aircraft carried more armor and fuel stores to better survivability and improved range.
The XB-28 "Dragon" (NA-63) (detailed elsewhere on this site) was an offshoot of the B-25 program. It was proposed to the USAAF through two completed prototypes as a high-altitude medium bomber to serve over the vast expanses of the Pacific Theater. The aircraft lost the trademark twin-tail rudders of the B-25 with a single rudder unit in its place. While proving an excellent entry when tested, the XB-28 was not adopted due to several factors - including the American switch to low-level bombing.
The B-25H was an improved form of the G-model. Two additional 0.50 cal HMGs were added to the nose. Before long, twin-gunned gun packs were added to the forward fuselage sides adding four more 0.50 caliber HMGs to the mix. While fixed to fire only forward, these guns could prove highly lethal to anything unfortunately caught in its path. The dorsal turret was pushed forward on the fuselage spine to provide for better views. The original M4 autocannon was succeeded by the developmental T13E1 model. Total production netted another 1,000 Mitchells.
The B-25H was crewed by six personnel made up of two pilots, a navigator (doubling as the bombardier), a dorsal turret gunner (doubling as the flight engineer), a radio operator (doubling as a beam gunner) and a tail gunner. Structurally the aircraft has a running length of 52.10 feet, a wingspan of 67.6 feet, and a height of 16.3 feet. Empty weight was 19,500lb against an MTOW of 35,000lb. Power was from 2 x Wright R-2800-92 "Twin Cyclone" 14-cylinder air-cooled radial piston engines outputting 1,700 horsepower each and used to drive three-bladed propellers. Performance included a maximum speed of 272 miles per hour, a cruising speed of 230 miles per hour, a range of 1,350 miles and a service ceiling up to 24,200 feet. Armament ranged from 12 to 18 machine guns of the .50 caliber variety as well as the aforementioned 75mm autocannon. Beyond the 3,000lb of conventional drop stores held internally, the bomber could be equipped with external shackles for carrying and releasing Mark 13 series torpedoes. Beyond this, the wings could field eight 5" HVAR rockets fro ground/ship attacks.
The B-25J was a meshing of D- and H-model qualities to serve as either in medium bomber or gunship roles. The mark was produced at the Kansas City location and could carry up to eighteen forward-facing machine guns for ground attack sorties or be used in the traditional bombing role. 4,318 of the type were ultimately built and about 316 were shipped to the British where they were known as the "Mitchell Mk.III". The J-model was the most produced Mitchell in the entire family line.
Other Mitchell Forms
The Mitchell series also included non-combat forms such as the CB-25J which was modified for the transport role. Similarly, the VB-25J was outfitted to serve in the military VIP transport role. The airframe also proved suitable as pilot, bombardier, navigator, gunnery and crew trainers through the TB-25 variant series which encompassed TB-25D through TB-25N. The United States Navy and Marine Corps also made use of the medium bomber in various guises: the PBJ-1C was modified to serve as an anti-submarine platform complete with airborne search radar fitted. The PBJ-1J was a Navy/Marine mark suitable for submarine hunting and carrying radar and rockets.
The B-25 saw widespread service across the globe, both in wartime and in the post-war world. Operators ranged from Argentina and Australia to Uruguay and Venezuela. Brazil, Canada, the Republic of China (Taiwan), France, Poland and the Soviet Union all fielded some form of the bomber or another. The RAF alone operated over 700 B-25s for their part in the story across nine total squadrons.
The B-25 In Service
The B-25 series proved its worth in combat all over the globe during World War 2. Like other bombers of the period, it could take an unbelievable amount of punishment and remain airborne. It was capable of flying on one engine and was noted for its excellent handling characteristics. The aircraft was a viable candidate for a plethora of sanctioned and unsanctioned conversions leading to a myriad of official and unofficial variants being had. The tricycle undercarriage, coupled with the heavily glazed and stepped cockpit, provided excellent vision out-of-the-cockpit for the pilots during landing and take-off actions.
The End of the Road
Like other wartime aircraft - even classic ones remembered to this day - the Mitchell series was quickly given up by the Americans with the close of the war. By 1947, just a few hundred examples remained in now-USAF service. Those that managed existences in American service into the 1950s were used solely for training and second-line roles before ultimately being passed on to Air National Guard units and th elike. The final USAF B-25 was retired in 1960. Other national powers continued to field the B-25 until the late 1970s.