North American B-25 Mitchell

The B-25 was probably the most successful allied medium bomber of the Second World War. Nearly 11,000 were built during the war, serving with most allied air forces, including the RAF and the Soviet Air Force. The vast majority served with the U.S.A.A.F. in the Pacific, where the Mitchell became a devastating ground attack aircraft.

The B-25 was the product of North American Aviation’s third attempt to win an Army bomber contract. In 1936 their NA-21 prototype has lost out to the Douglas B-18 Bolo. 1938 saw the development of the NA-40, the direct predecessor of the B-25. This time the Douglas A-20 Havoc won the contract. Finally, in March 1939, with war looming in Europe, the Army requested a five man medium bomber. This time, the North American NA-62 won the contract, getting the official designation B-25.

The outbreak of war in Europe increased the pace of aircraft development. The normal procedure for new Army aircraft was to build an experimental “X” version, then a prototype “Y” version, and then the production aircraft. With the B-25 it was decided to use the first production aircraft to test the design, and on 10 August 1939 an order was placed for 184 B-25 bombers. Once it entered full production, the B-25 was named the Mitchell, in honour of General William S. “Billy” Mitchell, an early supporter of air power in the American Army.

Variants

The B-25 constantly evolved throughout its production run. As production numbers increased, this resulted in a situation where the last version of most models was significantly different to the first.

B-25

The first twenty four aircraft built had no version letter. They are sometimes referred to as the B-25 NA to avoid confusion. The first nine aircraft produced did not have the B-25’s most distinctive feature – the gull wing. Instead, the wing continued to rise towards the wing tips. This was changed when the aircraft demonstrated “directional instability” - i.e. it was quite hard to fly in a straight line. The famous ‘gull wing’ design was introduced to correct this problem. The first aircraft flew in August 1940.

The B-25-NA was armed with .30 calibre machine guns – one in the nose, to be fired by the bombardier, three in the waist (top, left and right), and one .50 calibre gun in the tail, all to be operated by the radio operator.

The crew were split into three stations – bombardier in the nose compartment, pilot, co-pilot and navigator in the main cockpit and the radio operation behind the bomb bay.

B-25A

The next forty aircraft were designated B-25A. There were two main changes in this version – the pilot, co-pilot and bombardier were provided with armour behind their seats, and self-sealing fuel tanks were installed. Self sealing fuel tanks reduced the range of any aircraft that used them, as they reduced the fuel capacity – in the case of the B-25 from 913 gallons down to 694 gallons, but early wartime experience had proved them essential. This was the first version of the B-25 that was issued to operational units, joining the 17th Bombardment Group at the start of 1941, where ironically it replaced the B-18 Bolo.

B-25B


North American B-25B Mitchell
: Front Plan

The development of the B-25 was greatly aided by experience gained watching the war in Europe. The remaining 120 aircraft in the first production batch had their defensive armament increased. The three waist guns and one rear gun were replaced by two turrets, top and bottom, each mounting two .50 calibre machine guns. The top turret was a standard powered bomber turret, with guns mounted in a glass dome. The lower turret was a more ambitious weapon. It was a solid, retractable turret, aimed from above using a periscope. This design of turret was retained in the B-25C/D but eliminated after that. The retractable design could jam in the down position, while aiming with the periscope was difficult at best, and impossible if the periscope got dirty. This was the version of the B-25 that took part in the Doolittle raid over Japan. The last aircraft from the first batch was delivered in January 1942.

B-25C/D

North American B-25C Mitchell
North American B-25C Mitchell

This was the first version to be produced under wartime conditions, and was thus produced in much larger numbers than the earlier models. In total 1,625 B-25Cs and 2,290 B-25Ds were produced. The only difference between the two versions was that the C was built in North America’s factory at Inglewood, California, while the D was built at a government owned factory in Kansas City, Missouri. Both versions were known as the Mitchell II in RAF service (The US Navy called them the PBJ-1C and PBJ-1D). Initial changes from the B were relatively minor. The engine was changed to the Wright Cyclone R-2600-13, which used a different carburettor to the earlier engine. The aircraft was also equipped with a cabin heater, improved instruments and a high pressure brake system. The relatively minor changes from the B-25B allowed the B-25C to enter production in January 1942, only two months after the first flight of its prototype. Production of the B-25C ended in May 1943, the B-25D in March 1944.

B-25 Mitchell of No.98 Squadron
Mitchell II of No.98 Squadron

More changes were introduced during the production run. In the rough order that these first appeared, the major changes were as follows: The fuel capacity was increased to 974 gallons. The navigator was given an astrodome instead of the flat window of earlier versions. The gun turrets were improved. Capacity for under-wing bomb racks was included as standard, with stronger wings to support the extra weight. The aircraft was given the ability to carry at 2,000 lb torpedo under the bomb bay. The calibre of the flexible gun was increased from .30 to .50. A second .50 calibre gun was added, firing forward and controlled by the pilot. A different windshield was introduced, designed to improve visibility. The capability to carry a 325 gallon fuel tank in the bomb bay was added. Even after an improvement had been introduced, not all aircraft automatically had it. In addition older aircraft were often upgraded to match the newer standards.

Further modifications took place at modification centres or in the field. These included the addition of tail or waist guns, either .30 calibre guns fitted in the existing windows, or larger .50 calibre guns positioned in specially cut windows. Attempts were made to add a tail gun, but there was only room for a single .50 calibre machine gun in this version.

The war in the Pacific made unexpected demands on the aircraft. Here the jungle meant that level bombing was not very effective. Low level strafing attacks were preferred. A number of B-25Cs and Ds had the bomb sight removed and extra .50 calibre machine guns fitted instead. More forward firing guns were attached to the side of the fuselage. This trend for increased firepower was to be improved on in later versions.

F-10

Forty-five B-25Ds were modified for duty as photographic reconnaissance aircraft, and given the designation F-10. These aircraft used the R-2600-29 engine. They had the gun turrets removed. Three K-17 cameras were fitted, one pointing straight down, the other two at oblique angles on the side of the nose. These aircraft were most often used for ground mapping, essential intelligence work.

B-25G

Avro Lancaster damaged over Dusselford, 22 April 1944The B-25G was a dramatically different aircraft in purpose. While earlier B-25s had been designed as level bombers, the B-25G was purpose built for low level strafing attacks, initially against Japanese shipping in the Pacific. The biggest changes were in the nose. The bomb aimer’s compartment with its distinctive windscreen was replaced by a solid nose, containing two .50 calibre machine guns, and a huge 75-mm M4 cannon. This gun was over nine feet long, and fired a twenty-six inch long shell weighing 20 lbs! Each shell had to be loaded manually by the navigator, resulting in a slow rate of fire. Only twenty-one rounds were carried.

400 B-25Gs were built from new, and another 68 produced from B-25C airframes. The cannon was found to be too slow when used against ground targets, and was sometimes replaced by two more .50 calibre machine guns. The B-25G was produced between May and August 1943.

B-25H

The B-25H was developed to solve some of the problems discovered with the B-25G. The earlier model had been particularly weak on defensive firepower, especially after the lower turret was removed. The answer was a complete re-gigging of the turrets. The upper turret was moved from the rear of the aircraft forward to the navigator’s old position. Proper waist guns were added towards the rear of the aircraft, which were manned by the radio operator, who could easily swap between the two guns now the turret was no longer in the way. A proper rear turret was provided, with two .50 calibre machine guns. Two more .50 calibre guns were installed in the nose. Finally, two and then four more guns were added in gun packs on the side of the aircraft. This made the B-25 a very powerful ground attack aircraft, capable of bringing ten .50 calibre machine guns and the 75mm cannon into action on a strafing run.

B-25J

This was the version produced in the largest number – over 4000 were completed before the end of the war. It was built in two main versions. The majority had the glass bombardier’s restored, marking a return to the bomber role. A smaller number had a solid nose containing eight .50 calibre machine guns, for use in the ground attack role. Both versions retained the defensive firepower of the B-25h, with two waist turrets, one forward turret and a rear turret. The bomb bay was altered to allow a third 1,000 pound bomb to fit. Most B-25Js were built with strap-on gun packs on the side of the fuselage, although these were often removed in the field. A variety of other combinations of guns were also fitted in the field to suit particular circumstances.

Combat

Doolittle Raid

B-25 takes off for Doolittle RaidThe B-25’s most famous exploit was the Doolittle Raid of April 1942. At a time when the war in the Pacific could hardly have been going worse for the allies, this daring raid provided a great boost to morale. On 18 April sixteen B-25Bs, led by Lt. Colonel Jimmy Doolittle, launched from the USS Hornet. Their target was mainland Japan. After the raid they would have to fly on to mainland China and hope to find somewhere friendly to land. All sixteen aircraft reached Japan, and bombed targets on the home islands. The aircraft had been forced to launch early, and so had to land where they could in China. Thirteen of the sixteen crews made it to safety, one crew was interned in Russia and two were captured by the Japanese. The raid did very little actual damage, but it forced a change in the Japanese war plan. Realising that their current defensive perimeter was not deep enough to prevent raids on the homeland, the Japanese decided to expand that perimeter. One of their new targets was Midway Island, where they were to suffer one of their most decisive defeats of the war, losing four aircraft carriers and many irreplaceable airmen.

Far East

B-25 Mitchell over the Marshall Islands
B-25 Mitchell over the Marshall Islands
The majority of B-25s in American service were used in the Pacific. It fought on New Guinea, in Burma and in the island hopping campaign in the central Pacific. It was in the Pacific that the aircraft’s potential as a ground attack aircraft was discovered and developed. The jungle environment reduced the usefulness of standard level bombing, and made low level attack the best tactic. The ever-increasing amount of forward firing guns was a response to this, making the B-25 a formidable strafing aircraft.

In Burma the B-25 was often used to attack Japanese communication links, especially bridges in central Burma. It was also used to help supply the besieged troops at Imphal in 1944.

In the Pacific the B-25 proved itself to be a very capable anti-shipping weapon, sinking many of the ships being used to reinforce the Japanese position. Later in the Pacific war the distance between islands limited the usefulness of the B-25, although it was used against Guam and Tinian. It was also used against Japanese-occupied islands that had been bypassed by the main campaign, as happened in the Marshal Islands.

Middle East and Italy

The first B-25s arrived in Egypt just in time to take part in the battle of El Alamein. From there the aircraft took part in the rest of the campaign in North Africa, the invasion of Sicily and the advance up Italy. In Italy the B-25 was used in the ground attack role, concentrating on attacks against road and rail links in Italy, Austria and the Balkans. The B-25 had a longer range than the A-20 Havoc and A-26 Invaders, allowing it to reach further into occupied Europe. The five bombardment groups that used the B-25 in the desert and Italy were the only US units to use the B-25 in Europe.

Europe

The U.S. Eighth Air Force, based in Britain, concentrated on long range raids over Germany and occupied Europe. Although it did have a small number units equipped with twin engined aircraft, the B-25 was not amongst them. However, the RAF received nearly nine hundred Mitchells, using them to replace Douglas Bostons, Lockheed Venturas and Vickers Wellington bombers. The Mitchell entered active RAF service on 22 January 1943. At first it was used to bomb strategic targets in occupied Europe. After the D-Day invasion the RAF used its Mitchells to support the armies in Europe, moving several squadrons to forward airbases in France and Belgium.

Stats

Stat

B

C/D

G

H

J

Engine

Wright R-2600-9

Wright R-2600-13

Wright R-2600-13

Wright R-2600-13

Wright R-2600-29

Max HP each

1,700

1,700

1,700

1,700

1,700

Sustained HP

1,350 at 13,000 feet

Ditto

Ditto

Ditto

Ditto

Max speed

300 mph at 15,000 feet

284 mph at 15,000 feet

281 mph at 15,000 feet

275 mph at 13,000 feet

272 mph at 13,000

Ceiling

23,500 feet

21,200 feet

24,300 feet

24,800 feet

24,500 feet

Range

1,300 miles

1,525 miles

1,525 miles

?

1,350 miles

Bomb load

3,000 lbs (standard)

3,200 lbs (standard)

Ground attack

Ground attack

3,000 lbs

Range and bomb load are of course related. For short range missions more weight could be carried.  The B-25B could actually carry 3,500lbs of bombs, but at a cost to speed and range. Speed also fell as the weight of defensive firepower being carried increased, while the power provided by the Wright R-2600 engine remained the same.

PBJ Mitchell Units of the Pacific War, Jerry Scuts Osprey Combat Aircraft 40. This entry in the Combat Aircraft series looks at the seven Marine Corps squadrons to operate the Mitchell in the Pacific theatre, starting in March 1944. The small number of units involved means that this book looks at each of their wartime careers in some detail. [see more]
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How to cite this article: Rickard, J (28 March 2007), North American B-25 Mitchell, http://www.historyofwar.org/articles/weapons_B-25_Mitchell.html

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