North American B-25H Mitchell (NA-98)
WWII Twin-engine medium bomber

Archive Photos

North American B-25H-10 Mitchell (NA-98, AF 43-4899, c/n 98-21900, N37L) on display (9/29/2003) at the Kalamazoo Aviation History Museum, Portage, Michigan (Photos by John Shupek)

Overview

North American B-25/PBJ/Mitchell Series
• Role: Medium bomber
• Manufacturer: North American Aviation
• Designed by: John Leland "Lee" Atwood
• First flight: 19 August 1940
• Introduction: 1941
• Retired: 1979 (Indonesia)
• Primary users: United States Army Air Forces, Royal Canadian Air Force, Royal Air Force, Soviet Air Force
• Number built: 9,984
• Developed from: XB-21
• Variants: North American XB-28

The North American B-25 Mitchell was an American Twin-engine medium bomber manufactured by North American Aviation. It was used by many Allied air forces, in every theater of World War II, as well as many other air forces after the war ended, and saw service across four decades.

The North American B-25 was named in honor of General Billy Mitchell, a pioneer of U.S. military aviation. The North American B-25 is the only American military aircraft named after a specific person. By the end of its production, nearly 10,000 B-25s in numerous models had been built. These included a few limited variations, such as the US Navy’s and US Marine Corps’ North American PBJ-1 patrol bomber and the Army Air Forces’ North American F-10 photo reconnaissance aircraft.

Design and Development

The North American B-25 Mitchell was a descendant of the earlier XB-21 (North American-39) project of the mid-1930s. Experience gained in developing that aircraft was eventually used by North American in designing the North American B-25 Mitchell (called the NA-40 by the company). One North American NA-40 was built, with several modifications later being done to test a number of potential improvements. These improvements included Wright R-2600 radial engines, which would become standard on the later North American B-25 Mitchell’s.

In 1939, the modified and improved North American NA-40B was submitted to the United States Army Air Corps for evaluation. This aircraft was originally intended to be an attack bomber for export to the United Kingdom and France, both of which had a pressing requirement for such aircraft in the early stages of World War II. However, those countries changed their minds, opting instead for the also-new Douglas DB-7 (later to be used by the US as the A-20 Havoc). Despite this loss of sales, the North American NA-40B re-entered the spotlight when the Army Air Corps evaluated it for use as a medium bomber. Unfortunately, the North American NA-40B was destroyed in a crash on 11 April 1939. Nonetheless, the type was ordered into production, along with the Army’s other new medium bomber, the Martin B-26 Marauder.

Early Production

An improvement of the North American NA-40B, dubbed the North American NA-62, was the basis for the first actual North American B-25. Due to the pressing need for medium bombers by the Army, no experimental or service-test versions were built. Any necessary modifications were made during production runs, or to existing aircraft at field modification centers around the world.

A significant change in the early days of North American B-25 production was a re-design of the wing. In the first nine aircraft, a constant-dihedral wing was used, in which the wing had a consistent, straight, slight upward angle from the fuselage to the wing tip. This design caused stability problems, and as a result, the dihedral angle was nullified on the outboard wing sections, giving the North American B-25 its slightly gull wing configuration. Less noticeable changes during this period included an increase in the size of the tail fins and a decrease in their inward cant. A total of 6,608 North American B-25 Mitchell’s were built at North American’s Fairfax Airport plant in Kansas City, Kansas.

Operational History

The B-25 first gained fame as the bomber used in the 18 April 1942 Doolittle Raid, in which sixteen North American B-25B Mitchells led by the legendary Lieutenant Colonel Jimmy Doolittle, attacked mainland Japan four months after the bombing of Pearl Harbor. The mission gave a much-needed lift in spirits to the Americans, and alarmed the Japanese who believed their home islands were inviolable by enemy troops. While the amount of actual damage done was relatively minor, it forced the Japanese to divert troops for the home defense for the remainder of the war. The raiders took off from the carrier USS Hornet and successfully bombed Tokyo and four other Japanese cities without loss. However, 15 subsequently crash-landed en route to recovery fields in Eastern China. These losses were the result of the task force being spotted by Japanese fishing vessels forcing the bombers to take off 170 mi (270 km) early, fuel exhaustion, stormy nighttime conditions with zero visibility, and lack of electronic homing aids at the recovery bases. Only one landed intact; it came down in the Soviet Union, where its five-man crew was interned and the aircraft confiscated. Of the 80 aircrew, 69 survived their historic mission and eventually made it Back to American lines.

Following a number of additional modifications, including the addition of Plexiglas windows for the navigator and radio operator, heavier nose armament, and deicing and anti-icing equipment, the North American B-25C Mitchell was released to the Army. This was the second mass-produced version of the Mitchell, the first being the lightly-armed North American B-25B Mitchell used by the Doolittle Raiders. The North American B-25C Mitchell and North American B-25D Mitchell differed only in location of manufacture: The B-25C’s were built at Inglewood, CA, the B-25Ds at Kansas City, Kansas. A total of 3,915 North American B-25Cs and North American B-25Ds were built by North American during World War II.

Although the North American B-25 Mitchell was originally designed to bomb from medium altitudes in level flight, it was used frequently in the Southwest Pacific theater (SWPA) on treetop-level strafing and parafrag (parachute-retarded fragmentation bombs) missions against Japanese airfields in New Guinea and the Philippines. These heavily-armed Mitchells, field-modified by Major Paul I. "Pappy" Gunn, were also used on strafing and skip-bombing missions against Japanese shipping trying to re-supply their land-based armies. Under the leadership of Lieutenant General George C. Kenney, North American B-25s of the Fifth and Thirteenth Air Forces devastated Japanese targets in the SWPA from 1942 to 1945, and played a significant role in pushing the Japanese Back to their home islands. North American B-25s were also used with devastating effect in the Central Pacific, Alaska, North Africa, Mediterranean and China-Burma-India (CBI) theaters.

Because of the urgent need for hard-hitting strafer aircraft, a version dubbed the North American B-25G was developed, in which the standard-length transparent nose and the bombardier were replaced by a shorter solid nose containing two fixed 0.50 in (12.7 mm) machine guns and a 75 mm (2.95 in) M4 cannon, one of the largest weapons fitted to an aircraft, similar to the experimental British Mosquito Mk.XVIII, and German Ju.88P heavy cannon carrying aircraft. The cannon was manually loaded and serviced by the navigator, who was able to perform these operations without leaving his crew station just behind the pilot. This was possible due to the shorter nose of the G-model and the length of the M4, which allowed the breech to extend into the navigator’s compartment.

The North American B-25G’s successor, the North American B-25H Mitchell, had even more firepower. The M4 gun was replaced by the lighter T13E1, designed specifically for the aircraft. The 75 mm (2.95 in) gun fired at a muzzle velocity of 2,362 ft/s (about 720 m/s). Due to its low rate of fire (approximately four rounds could be fired in a single strafing run) and relative ineffectiveness against ground targets, as well as substantial recoil, the 75 mm (2.95 in) gun was sometimes removed from both G and H models and replaced with two additional .50 in (12.7 mm) machine guns as a field modification. The North American B-25H also mounted four fixed forward-firing 0.50 (12.7 mm) machine guns in the nose, four more fixed ones in forward-firing cheek blisters, two more in the top turret, one each in a pair of new waist positions, and a final pair in a new tail gunner’s position. Company promotional material bragged the North American B-25H could "bring to bear 10 machine guns coming and four going, in addition to the 75 mm cannon, a brace of eight rockets and 3,000 lb (1,360 kg) of bombs."

The North American B-25H Mitchell also featured a redesigned cockpit area, with the top turret moved forward to the navigator’s compartment (thus requiring the addition of the waist and tail gun positions), and a heavily modified cockpit designed to be operated by a single pilot, the co-pilot’s station and controls deleted, and the seat cut down and used by the navigator/cannoneer, the radio operator being moved to the aft compartment, operating the waist guns. A total of 1,400 North American B-25G Mitchells and North American B-25H Mitchells were built in all.

The final version of the Mitchell, the North American B-25J Mitchell, looked much like the earlier B-25B, B-25C and B-25D, having reverted to the longer nose. The less-than-successful 75 mm (2.95 in) cannon was deleted on the B-25J model. Instead, 800 of this version were built with a solid nose containing eight 0.50 (12.7 mm) machine guns, while other B-25J Mitchells featured the earlier greenhouse style nose containing the bombardier’s position. Regardless of the nose style used, all B-25J Mitchells also included two 0.50 in (12.7 mm) guns in a fuselage package located directly under the pilot’s station, and two more such guns in an identical package just under the co-pilot’s compartment. The solid-nose North American B-25J Mitchell variant carried an impressive total of 18 0.50 in (12.7 mm) guns: eight in the nose, four in under-cockpit packages, two in an upper turret, two in the waist, and a pair in the tail. No other bomber of World War II carried as many guns. However, the first 555 North American B-25J Mitchells (the North American B-25J-1-NC Mitchell production block) were delivered without the fuselage package guns, because it was discovered muzzle blast from these guns was causing severe stress in the fuselage; while later production runs returned these guns, they were often removed as a field modification for the same reason. In all, 4,318 North American B-25J Mitchells were built.

The North American B-25 Mitchell was a safe and forgiving aircraft to fly. With an engine out, 60° banking turns into the dead engine were possible, and control could be easily maintained down to 145 mph (230 km/h). However, the pilot had to remember and maintain engine-out directional control at low speeds after take off with rudder - if this was attempted with ailerons, the aircraft would snap out of control. The tricycle landing gear made for excellent visibility while taxiing. The only significant complaint about the North American B-25 Mitchell was the extremely high noise level produced by its engines; as a result, many pilots eventually suffered from various degrees of hearing loss.

The North American B-25 Mitchell was also an amazingly sturdy aircraft and could withstand tremendous punishment. One well-known North American B-25C Mitchell of the 321st Bomb Group was nicknamed "Patches" because its crew chief painted all the aircraft’s flak hole patches with high-visibility zinc chromate paint. By the end of the war, this aircraft had completed over 300 missions, was belly-landed six times and sported over 400 patched holes. The airframe was so bent, straight-and-level flight required 8° of left aileron trim and 6° of right rudder, causing the aircraft to "crab" sideways across the sky.

An interesting characteristic of the North American B-25 Mitchell was its ability to extend range by using one-quarter wing flap settings. Since the aircraft normally cruised in a slightly nose-high attitude, about 40 gal (150 L) of fuel was below the fuel pickup point and thus unavailable for use. The flaps-down setting gave the aircraft a more level flight attitude, which resulted in this fuel becoming available, thus slightly extending the aircraft’s range.

By the time a separate United States Air Force was established in 1947, most North American B-25 Mitchells had been consigned to long-term storage. However, a select number continued in service through the late 1940s and 1950s in a variety of training, reconnaissance and support roles. Its principal use during this period was for undergraduate training of multi-engine aircraft pilots slated for reciprocating engine or turboprop cargo, aerial refueling or reconnaissance aircraft. Still others were assigned to units of the Air National Guard in training roles in support of Northrop F-89 Scorpion and Lockheed F-94 Starfire operations. The final example of a North American B-25 Mitchell was struck from the active USAF roles in January 1959. Today, many North American B-25 Mitchells are kept in airworthy condition by air museums and collectors.

Empire State Building Incident

On Saturday, 28 July 1945, at 0940 (while flying in thick fog), a USAAF North American B-25D Mitchell crashed into the north side of the Empire State Building, hitting between the 79th and 80th floor. Fourteen people were killed - 11 in the building, along with Colonel William Smith and the other two occupants of the bomber. It was partly because of this incident that towers 1 and 2 of the World Trade Center were designed to withstand the impact of a Boeing 707 aircraft.

Variants

Bomber Variants

• B-25: The first version of the B-25 delivered. No prototypes were ordered. The first nine aircraft were built with constant dihedral angle. Due to low stability, the wing was redesigned so that the dihedral was eliminated on the outboard section. (Number made: 24.)
• B-25A: Version of the B-25 modified to make it combat ready; additions included self-sealing fuel tanks, crew armor, and an improved tail gunner station. No changes were made in the armament. Re-designated obsolete (RB-25A designation) in 1942. (Number made: 40.)
• B-25B: Rear turret deleted; manned dorsal and remotely-operated ventral turrets added, each with a pair of 0.50 in (12.7 mm) machine guns. The ventral turret was retractable, but the increased drag still reduced the cruise speed by 30 mph (48 km/h). 23 were delivered to the RAF as the Mitchell Mk I. The Doolittle Raiders flew B-25B’s on their famous mission. (Number made: 120.)
• B-25C: Improved version of the B-25B: power plants upgraded from Wright R-2600-9 radials to R-2600-13s; de-icing and anti-icing equipment added; the navigator received a sighting blister; nose armament was increased to two .50 in (12.7 mm) machine guns, one fixed and one flexible. The B-25C model was the first mass-produced B-25 version; it was also used in the United Kingdom (as the Mitchell II), in Canada, the People’s Republic of China, the Netherlands, and the Soviet Union. First mass-produced B-25 model. (Number made: 1,625.)
• ZB-25C:
• B-25D: Identical to the B-25C, the only difference was that the B-25D was made in Kansas City, Kansas, whereas the B-25C was made in Inglewood, CA. First flew on 3 January 1942. (Number made: 2,290.)
• ZB-25D:
• XB-25E: Single B-25C modified to test de-icing and anti-icing equipment that circulated exhaust from the engines in chambers in the leading and trailing edges and empennage. The aircraft was tested for almost two years, beginning in 1942; while the system proved extremely effective, no production models were built that used it prior to the end of World War II. Many prop aircraft today use the XB-25E system. (Number made: 1, converted.)
• ZXB-25E:
• XB-25F-A: Modified B-25C that tested the use of insulated electrical de-icing coils mounted inside the wing and empennage leading edges as a de-icing system. The hot air de-icing system tested on the XB-25E was more practical. (Number made: 1, converted.)
• XB-25G: Modified B-25C in which the transparent nose was replaced by a solid one carrying two fixed 0.50 in (12.7 mm) machine guns and a 75 mm (2.95 in) M4 cannon, then the largest weapon ever carried on an American bomber. (Number made: 1, converted.)
• B-25G: To satisfy the dire need for ground-attack and strafing aircraft, the B-25G was made following the success of the prototype XB-25G. The production model featured increased armor and a greater fuel supply than the XB-25G. One B-25G was passed to the British, who gave it the name Mitchell II that had been used for the B-25C. (Number made: 420.)
• B-25H: An improved version of the B-25G. It featured two additional fixed .50 in (12.7 mm) machine guns in the nose and four in fuselage-mounted pods; the heavy M4 cannon was replaced by a lighter 75 mm (2.95 in) T13E1. (Number made: 1,000; number left flying in the world: 1.)
• B-25J: The last production model of the B-25, often called a cross between the B-25C and the B-25H. It had a transparent nose, but many of the delivered aircraft were modified to have a solid nose. Most of its 14 to 18 machine guns were forward-facing for strafing missions. 316 were delivered to the Royal Air Force as the Mitchell III. (Number made: 4,318.)
• CB-25J: Utility transport version.
• VB-25J: A number of B-25s were converted for use as staff and VIP transports. Henry H. Arnold and Dwight D. Eisenhower both used converted B-25Js as their personal transports.

Trainer Variants

Most models of the North American B-25 Mitchells were used at some point as training aircraft.

• TB-25D: Originally designated AT-24A (Advanced Trainer, Model 24, Version A). Trainer modification of B-25D. In total, 60 AT-24s were built.
• TB-25G: Originally designated AT-24B. Trainer modification of B-25G.
• TB-25C: Originally designated AT-24C. Trainer modification of B-25C.
• TB-25J: Originally designated AT-24D. Trainer modification of B-25J. Another 600 B-25J’s were modified after the war.
• TB-25K: Hughes E1 fire-control radar trainer (Hughes). (Number made: 117.)
• TB-25L: Hayes pilot-trainer conversion. (Number made: 90.)
• TB-25M: Hughes E5 fire-control radar trainer. (Number made: 40.)
• TB-25N: Hayes navigator-trainer conversion. (Number made: 47.)

USN/USMC Variants

• PBJ-1C: Similar to the B-25C for the US Navy; often fitted with airborne search radar and used in the anti-submarine role.
• PBJ-1D: Similar to the B-25D for the US Navy and US Marine Corps. Differed in having a single 0.50 in (12.7 mm) machine gun in the tail turret and beam gun positions similar to the B-25H. Often fitted with airborne search radar and used in the anti-submarine role.
• PBJ-1G: US Navy/US Marine Corps designation for the B-25G.
• PBJ-1H: US Navy/US Marine Corps designation for the B-25H.
• PBJ-1J: US Navy designation for the B-25J-NC (Blocks -1 through -35) with improvements in radio and other equipment. Often fitted with "package guns" and wingtip search radar for the anti-shipping/anti-submarine role.

North American B-25 Mitchell Production

Operators

• Australia: Royal Australian Air Force, No. 2 Squadron RAAF
• Argentina
• Biafra: Biafran Air Force operated two aircraft.
• Bolivia
• Brazil: Brazilian Air Force (75 units)
• Canada: Royal Canadian Air Force
• China
• People’s Republic of China: People’s Liberation Army Air Force operated captured Nationalist Chinese aircraft.
• Chile
• Colombia
• Cuba
• Dominican Republic
• France
• Indonesia: Indonesian Air Force received some B-25 Mitchell’s from Netherlands, the last example retired in 1979.
• Mexico
• Netherlands: Royal Netherlands Air Force, No. 18 (Netherlands East Indies) Squadron RAAF, No. 119 (Netherlands East Indies) Squadron RAAF, No. 320 Squadron, Royal Dutch Naval Air Service
• Peru
• Poland: Polish Air Forces on exile in Great Britain, No. 305 Polish Bomber Squadron
• Spanish State: Spanish Air Force
• Soviet Union: Soviet Air Force received a total of 866 B-25s (of types C/D/S/G/J).
• United Kingdom: Royal Air Force received more than 900 aircraft.
• United States: United States Army Air Forces, United States Navy, United States Marine Corps
• Uruguay
• Venezuela

Specifications (B-25H)

Type

• Twin-engine medium bomber.

Wings

• Mid-wing cantilever monoplane of all-metal construction.
• Wing in five sections consisting of a two-spar center-section permanently attached to the fuselage, two outer single-spar sections and two detachable wing-tips.
• Fuel and oil tanks integral with the center-section structure.
• Outer wings have sealed compartments for flotation purposes.
• Ailerons of sealed type, and are fitted with fixed and controllable trimming-tabs.
• Hydraulically operated slotted trailing-edge flaps inboard of ailerons and divided by tails of engine nacelles.
• Flaps have fairings which hinge upward into the wings to form a continuous slot opening when the flaps are lowered.

Fuselage

• Semi-monocoque four-longeron structure of aluminum-alloy with covering of same material.
• That portion of the fuselage above the bottom surface of the center-section and between the front spar and trailing-edge is permanently attached to and removable with the center-section.

Tail Unit

• Cantilever monoplane type with twin fins and rudders.
• Elevators and rudders have fixed and controllable trimming-tabs.

Landing Gear

• Tricycle type with all wheels fully retractable.
• All wheels retract aft, the main wheels into the engine nacelles in the nosewheel and tail-skid into the fuselage.
• Doors cover all openings in both retracted and extended positions.
• Hydraulic retraction, with a mechanically-operated emergency system.
• The swiveling nose-wheel has shimmy damper and centering device and lock.
• Main wheels have hydraulic brakes.

Power Plant

• Two 1,700-hp Wright "Cyclone" R-2600-13 fourteen-cylinder two-row radial air-cooled engines with two-speed superchargers in semi-monocoque nacelles mounted below the extremities of the center-section.
• Three-bladed Hamilton-Standard constant-speed full-feathering airscrews with anti-icers.
• Each engine fitted with independent fuel system consisting of two interconnected fore and aft compartments fitted with bullet-proof self-sealing fuel cells.
• All fuel lines in wings and fuselage are of self-ceiling type.
• Total normal fuel capacity 646 US gallons (2,445.4 liters).
• Additional droppable 420 US gallons tank (1,589.9 liters) may be installed in bomb-bay.
• Each engine has independent oil system, each oil compartment being built semi-integral with center-section directly aft the fireproof bulkhead.

Accommodation

• Crew of five, comprising pilot, second pilot, bombardier, radio-operator and gunner.
• All members of crew may interchange position in the air.
• Bomb-aimer’s position in transparent nose.
• Pilot’s compartment seating two side-by-side forward of the plane of the airscrews.
• Access below pilot’s seats to navigator’s compartment which is just ahead of the bomb-bay.
• Passageway over bomb-bay leads to radio-operator’s position and upper and lower gun positions.
• Further aft is located the photographic equipment, and in the extreme tail there is a prone observer’s position.
• All crew positions are armored.

Armament

• One 75-mm cannon and four 0.50-in machine guns in nose.
• Two 0.50-in machine guns in dorsal turret.
• Two 0.50-in machine guns, one in left and one in right waist positions.
• Two 0.50-in machine guns in Bell tail turret.
• Bomb-bay for internal stowage of bombs or torpedo beneath center-section.
• Bomb load: 3,000 lb (1,360.78 kg)

Dimensions

• Wingspan: 67 ft 6.7 in (20.59 m)
• Length: 51 ft 0 in (15.54 m)
• Height: 15 ft 9.2 in (4.81 m)
• Wing area: 609.7 ft² (56.64 m²)

Weights

• Weight empty: 19,000 lb (8,618.26 kg)
• Weight loaded: 35,000 lb (15,875.73 kg)
• Fuel: 974 to 1,624 US-gal (3,686.99 to 6,147.51 liters)

Performance

• Cruising speed: 244 mph (392.68 km/h)
• Maximum speed: 280 mph (450.62 km/h)
• Landing speed: 97 mph (156.11 km/h)
• Climb rate: 800 ft/min (243.84 m/min)
• Service ceiling: 24,600 ft (7,498.08 m)
• Range: 1,500 miles (2,414.02 km)

References

• Wikipedia: North American B-25 Mitchell
• Three-View Drawing and Photos: John Shupek
• Avery, Norm, "North American NA-40/-40B / NA-62/-108 B-25/A/B/C/D/G/H/J / F-10 /AT-24 / PBJ Mitchell," North American Aircraft 1934-1998, Volume 1, Santa Ana, CA: Narkiewicz//Thompson, 1998, ISBN 0-913322-05-9, pp 63-91
• Bridgman, Leonard (editor). "The North American Mitchell," Jane’s All The World’s Aircraft 1943-44, London: Sampson Low, Marston & Company, Ltd., 1944, pp 216c - 218c