Fighting the U-boats
North American B-25 Mitchell
prepared by Emmanuel Gustin
The North American B-25 Mitchell was the most built medium bomber of the Second World War. The B-25 was versatile and effective, and flew on all fronts. 9816 were built. About 20 countries used the B-25, including a number that acquired B-25s after the war.
The story of the B-25 began in 1938, when the USAAF asked for proposals for a twin-engined medium bomber, able to carry 1200lb (544kg) of bombs over 1200 miles (1930km), at a speed of at least 200mph (321km/h). One of the responses was the North American NA-40 design, a high wing aircraft with twin tailfins, a glazed nose, a long greenhouse cockpit, and tricycle landing gear. The NA-40 was a three-seater, armed with seven .30 machineguns, four fixed guns in the wings and three flexible ones. It was powered by two 1100hp Pratt & Whitney R-1830 Twin Wasp radials. The prototype made its first flight on 10 February 1939, flown by Paul Balfour. The NA-40 was a good aircraft, but to improve performance the engines were replaced by 1350 hp Wright GR-2600-A71 Cyclones. In March the NA-40B was handed over to the USAAC for evaluation, but two weeks later the aircraft was destroyed in a crash.
Nevertheless the Air Corps was favourably impressed, and it asked for an improved model. North American responded with a completely new aircraft, the NA-62, which was based on the experience with the NA-40 but retained little more than its general configuration. The fuselage was made longer, deeper and wider, so that the bomb load could be doubled. The wing was lowered to a mid-wing position, the engine nacelles were refined, and the greenhouse cockpit replaced by a more conventional cockpit deck. The crew was increased to five, including two pilots. The engines were now 1700hp R-2600-9 radials. The NA-40 had looked practical, but the highly polished NA-62 was a very elegant aircraft. It was an elegance that combat modifications would soon destroy. The Army accepted it as the B-25 in September 1939, and ordered 184. Vance Breese made the first flight on 19 August 1940. The aircraft was soon named Mitchell, in honor of General Billy Mitchell, the controversial, brilliant man who had fought for an independent air force with a strong bomber component.
There soon were a number of modifications to enhance controllability. The tail surfaces were enlarged, and from the 10th production aircraft onwards, the wing dihedral on the outer wing panels was eliminated. This gave the B-25 its characteristic gull-winged look.
Most modifications to the B-25 were changes in its armament. The B-25 had appeared with with hand-held .30 guns in the nose, the sides of the fuselage, and the top of the fuselage. A .50 gun was installed in the tail. It lacked armour and self-sealing fuel tanks. Reports of combat experience in Europe indicated that this was not good enough. Only 24 B-25s were delivered before production switched to the B-25A.
The B-25A, with armour and self-sealing fuel tanks, appeared in mid-1941. The last 120 of the initial contract were completed as B-25Bs, with power-operated dorsal and ventral turrets containing two .50 guns each. The tail gun was eliminated. However, the remotely-operated ventral turret was considered almost worthless, and usually deleted in the field. The operator had to look through a periscope, and his narrow view made him disoriented and even airsick. Often a pair of broomsticks were fitted in the transparent tailcone to deter stern attacks. Maximum speed was now down to 482km/h, compared with 518km/h on the B-25. The B-25C introduced R-2600-13 engines, and an enlarged bomb bay with fittings for a bomb bay fuel tank. Top speed now dropped to 457km/h. Later wing racks for bombs and fuselage shackles for a torpedo were added. The B-25D was equivalent with the B-25C, but built in Dallas instead of Inglewood. With respectively 1619 and 2290 built, the B-25C and B-25D were the first major production models. A number where completed as F-10 reconnaissance aircraft, with three cameras in the nose.
In Australia another modification was pioneered. There it was observed that low-level attacks were the most effective, both against land targets and against ships. Against ships the B-25s used "skip bombing", dropping their bombs below 250ft (76m) and at 200mp (312km/h) so that they bounced over the surface of the water and hit ships on the waterline. But the B-25s needed more firepower to suppress the defensive fire. The locally engineered solution was to install additional fixed .50 guns in the nose, and add four .50 guns in packs attached to the fuselage sides. The plexiglass nose panels were simply painted over.
These developments were recognized by North American, and resulted in the B-25G. This model featured a short, blunt, metal-covered nose with two (later four) .50 guns. The most significant change was the installation of a 75mm M4 cannon in a tunnel, for wich the seat of the co-pilot had to be deleted. The M4 was 2.9m long and weighed 410kg. The navigator now had to load the 6.8kg rounds. The .50 nose guns were used for sighting, and then the 75mm cannon was fired. The cannon was effective against small ships and other targets, but because of its low rate of fire only four rounds could be fired in a typical attack. It was often deleted in favour of more .50 guns. The B-25H had the lighter TE13E1 cannon, and great changes were made to the defensive armament. The dorsal turret was moved forward, the ventral turret deleted, and a tail gunner with two .50 guns seated in a new tail gun position. Bulging side windows for beam guns were also added. Production of these two models was 405 and 1000, respectively.
The final production model was the B-25J. This had all refinements of the B-25H, but was delivered with two interchangeable nose cones. One was a standard bomber nose plexiglass panels for the bombardier, but also two fixed forward-firing guns. The alternative nose was a metal cone that was better streamlined than that used on the B-25G and contained eight .50 guns. Even this nose was often combined with the four guns packs on the fuselage sides, and the later B-25J models could carry four 5-inch rockets under each wing. The engines were now 1700hp R-2600-92s, but the increase in weight had now reduced speed to 438km/h. The B-25J remained in production from 1943 to 1945, and 4318 were delivered.
On the ground a B-25 could be difficult to handle, because its nosewheel was not connected to the controls, and the aircraft had to be steered with its harsh, powerful brakes. Despite its weight the B-25 had a good acceleration on take-off. But in level flight it became pedestrian, with heavy controls. Its +2.8g acceleration limit did not allow brusque manoeuvres. On the other hand, its great stability made it easy to fly hands-off. The proximity of the propellers to the cockpit made it extremely noisy. Landing characteristics were good, if the aircraft was allowed enough room to slow down. The Navy even conducted deck landing trails with a PBJ-1H (equivalent to B-25H). A "hooked" PBJ-1H landed on USS Shangri-La on 15 November 1944. Although tests were successful, no operational use was made of this technique.
The B-25 enters battle ...
The first unit to receive B-25s was the 17th Bombardment Group (Medium). After Pearl Harbour this moved to the west coast to fly submarine patrols, and on 24 December one sank a Japanese submarine. Later the 17th moved to the east coast, where they sank a German submarine. Other B-25s, including 40 originally built for the Dutch, were sent to the South-West Pacific area. Operating from bases in Australia and New Guinea, they tried to halt the Japanese advance. They were joined by a Dutch unit. Later other B-25s groups operated in the Central Pacific, and over the Aleutian islands. In June 1942, B-25s appeared in Burma, and in August 1942, the 12th Group began operations in North Africa, were they were joined by the 340th Group in March 1943. The USAAF observed that the faster B-26 was more suitable for operations in Europe, and operated the B-25 mainly in the Mediterranean and the Pacific.
As an early revenge for Pearl Harbour, one of the most unusual operations of the war was planned. Sixteen B-25Bs were modified by installing extra fuel tanks, for 4319 liters instead of the usual 2627. The volunteer crews received additional training. The bombers were loaded on the aircraft carrier USS Hornet, and on 18 April 1941, the bombers lead by Major J. Doolittle took off to their targets in Tokyo, Nagoya, Osaka and Kobe. The plan was to land in China, where the bombers would be a welcome reinforcement; but because USS Hornet was detected by the Japanese the aircraft had too take off early, and none reached its destination. Although damage was light and all aircraft were lost, the "Doolittle Raid" was a big boost for the American morale. It also was a serious concern for the Japanese, who became acutely aware that an American force could come to within strike distance of Japan almost undetected.
In 1942 a small number of B-25Bs was sent to Britain, which deployed them in North Africa and eventually in India. Operations in West Europe were postponed until the more capable B-25C arrived. The 2nd Group of Bomber Command flew its first mission on 22 January 1943, against oil storage facilities in Belgium. It soon adopted very tight formations for mutual support, and their aircraft retained the ventral turrets. The RAF continued to use the Mitchell until the end of the war, receiving B-25H and B-25J models. Total deliveries were 872, but a substantial number of these were sent to the Bahamas for anti-submarine patrols.
Some were also sent to the USSR. The USSR would receive 862 B-25s, and after the war the NATO allocated the type the recognition name Bank.
In the summer of 1942 the USAAF and the USN came to an agreement which allowed the Navy to operate land-based bombers. In early 1943 the Marine Corps began to receive its first B-25Cs, which were given the Navy designation PBJ-1C. On 17 March 1944 the "Flying Nightmares" of VMB-143 flew their first operational mission, against Rabaul. A total of 706 aircraft were delivered to the USN and USMC. These aircraft were painted grey, or later sea blue. Radar was installed, and depending on the type it was under the aft fuselage, on the wing tip, or in the nose. Their normal armament consisted of depth charges, but they could also carry a torpedo. Nine USMC squadrons used the PBJ-1 in combat.
At the end of the war the B-25 was rapidly replaced in the bomber role. In the late 1940s it was already obsolete, although the RCAF flew the B-25 in a combat role until the 1960s. But the aircraft were still around in large numbers. Already during the war, conversion of many to AT-24 or TB-25 crew trainers had begun. Typically, the gun turrets were deleted and the warpaint stripped off. Some bomber trainers had modern radars installed in their noses. Other B-25s were modified to become target tugs, transports or VIP transports. The last USAF Mitchell, a staff transport, was retired on 21 May 1960.
North American B-25H Mitchell
Crew of five.
|Engines:|| Two 1700hp Wright R-2600-13 Cyclone air-cooled radial engines.
|Wing span|| 20.60m, length 15.54m, height 4.80m, wing area 56.67m2.
|Empty weight|| 9061kg, max take-off weight 16351kg
|Maximum speed|| 443km/h at 3960m.
|Service ceiling|| 7255m.
|Normal range|| 2715km.
|Armament:||In the nose one 75mm T-13E1 cannon with 21 rounds, and four .50 guns. Four .50 guns in packs attached to the front fuselage. Two gun in a dorsal turret, two in the tail turret, and one in each beam position. Eight 5-inch rockets under the outer wing panels. 1360kg of bombs.|
- Medium with the mostest - The B-25 Mitchell
Jenny Scutts, in Air International, January and February 1993.
- Essai en Vol: Pascal Auchatraire et le B-25
Pascal Auchatraire, in Le Fana de l'Aviation, Janvier 1993.
- Twenty Faithful Years -- The B-25 in RCAF service
John Lyzun, in Air Enthusiast, July-August 1996.