<p>In an environment of thinly concealed disinterest in rapidly deteriorating world affairs during the mid-1930s, the US Army Air Corps possessed neither the political influence nor financial appropriations to undertake the scale of re-equipment or expansion being pursued in Europe. The turning point arrived on 28 September 1938 with a White House meeting at which President Roosevelt demanded massive expansion of the US Army Air Corps, prompting General Arnold to express a belief that his service had finally achieved its 'Magna Carta'.
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<p>At that time a US Army Air Corps requirement existed for a medium bomber, a design for which North American Aviation Inc. at Inglewood, California, had already started work at private expense. This NA-40 design was a twin-engined, three-seat shoulder-wing aircraft with tricycle landing gear, designed for two 1,100-hp (821-kW) Pratt & Whitney R-1830-56C3G radials. By chance this prototype was first flown by Paul Balfour in January 1939, the same month that the USAAC announced a competition for medium bomber designs to be submitted by 5 July that year. With engines changed to 1,350-hp (1,007-kW) Wright GR-2600-A71s, the prototype (now the NA-40B) was delivered for tests at Wright Field in March.
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<p>The NA-40B was destroyed in an accident after only two weeks at Wright Field, but had already so impressed USAAC pilots that North American was instructed to continue development, specifying a number of changes. A wider fuselage allowed the bomb load to be doubled; the wing was moved to mid-fuselage position and the top line of the cockpit was redrawn to be flush with the upper line of the fuselage; the crew was increased from three to five and the armament increased to three 0.3-in (7.62-mm) guns in nose, dorsal and ventral positions, and one 12.7-mm (0.5-in) gun in the extreme tail The new design was not completed until September 1939, but already North American had (on 10 August) received an $11,771,00 contract for 184 production aircraft.
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<hi style=hdr1>Flight Test Program</hi>
<p>The first aircraft was a static test airframe, completed in July 1940, and the first flight by a production B-25 took place on 19 August with 1,700-hp (1,268-kW) R-2600-9s at a gross weight of 27,310 lb (12,388 kg), an increase of more than 7,800 lb (3,538 kg) compared with the original NA-40 prototype. Early flight tests with the first B-25 disclosed a marked lack of directional stability, a shortcoming that was quickly and effectively cured by almost eliminating the dihedral of the wings outboard of the engines, a change that gave the B-25 its characteristic gull wing. Only the first nine B-25s (40-2165 to 402173) were completed with the straight dihedral wing, followed by 15 with the modified wing, no differentiation being made in the basic designation.
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<p>Early wartime combat experience in Europe had demonstrated the need for protection from gunfire, and 1941 saw the widespread modification of many American aircraft to benefit from this rather obvious lesson. The next B-25s on the Inglewood production line were 40 B-25As (NA-62A) with armor protecting the pilots and with self-sealing fuel tanks. First service deliveries were made in the spring of 1941 to the 17th Bombardment Group (Medium), commanded by Lieutenant Colonel (later Brigadier General) Walter R. Peck, whose component squadrons (the 34th, 37th and 95th) were based at Lexington County Airport, South Carolina. At the end of that year the group moved to the west coast of the USA for coastal anti-shipping patrols after the Japanese attack at Pearl Harbor, and one of its B-25As sank a Japanese submarine on 24 December.
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<p>Meanwhile, the original 1939 production contract was completed in 1941 with the production of 120 B-25Bs (although one aircraft, 402243, crashed and was written off before delivery). This version featured electrically operated Bendix dorsal and ventral turrets, each with a pair of 0.5-in (12.7-mm) machine guns; the tail gun was deleted, and the gross weight was increased to 28,460 lb (12,909 kg). With considerably accelerated production already planned for further improved versions, the American government was able to supply 23 B-25Bs to the UK under Lend-Lease. However, only one such aircraft (termed the Mitchell Mk I in the Royal Air Force) arrived in the United Kingdom, the remainder being shipped to North Africa. A small number of other B-25Bs were shipped to the North Russian ports as deck cargo in the early PQ convoys.
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<hi style=hdr1>US Diversions</hi>
<p>About 40 B-25Bs were scheduled for delivery to the Dutch in the Netherland East Indies but, as a result of the rapidly deteriorating situation facing the Americans in the southwest Pacific, all were diverted to the 13th and 19th Squadrons of the 3rd Bombardment Group, USAAF, at Brisbane, Australia, commanded by Robert F. Strickland (who rose from first lieutenant to full colonel in only nine months, in 1942). Other aircraft were delivered to the 17th Bombardment Group (Medium), replacing the earlier B-25As.
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<p>It was in April 1942 that the B-25 leapt into the headlines with one of the war's most daring epics, when 16 B-25Bs made an extraordinary attack on the Japanese mainland. Using aircraft from the 17th Group, modified to carry 1,141 US gal (4,319 liters) of fuel compared with the standard 694 US gal (2,627 liters), these B-25Bs were crewed by volunteers led by Lieutenant Colonel James H. Doolittle, one of America's greatest airmen who as long ago as 1925 had won a Schneider Trophy race, and who later in the war commanded the 8th, 12th and 15th US Air Forces. The B-25's ventral turrets and Norden bombsights were removed and two wooden 'guns' fitted in the tail for deception purposes, yet the gross weight rose to 31,000 lb (14,062 kg). Carried to within 800 miles (1,290 km) of the Japanese mainland aboard the carrier USS Hornet, Doolittle led his aircraft in low-level attacks on targets in Tokyo, Kobe, Yokohama and Nagoya on 18 April. All the raiders either crashed or force-landed, the majority of crews being repatriated by the Russians or Chinese. Doolittle was awarded the Medal of Honor on his return from this token but inspiring attack at a time when America's forces were still reeling from the Japanese onslaught after Pearl Harbor. At that time, the B-25 was the heaviest aircraft ever to have been launched from the deck of a carrier.
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<hi style=hdr1>Detail Improvements</hi>
<p>Extensive detail redesign identified the B-25C, of which deliveries started just before the end of 1941, 1,619 being produced at the parent plant at Inglewood and 2,290 (designated B-25D) at a second North American factory at Dallas, Texas. The aircraft were powered by 1,700-hp (1,268-kW) R-2600-13s and were equipped with autopilots; external racks on the fuselage could increase the fuel capacity to 1,100 US gal (4,164 liters), while this and a bomb bay fuel tank carrying 585 US gal (2,214 liters) took the maximum gross weight to 41,800 lb (18,960 kg). Maximum bomb load of 5,200 lb (2,359 kg) comprised 3,200 lb (1,452 kg) internally plus eight 250-lb (113-kg) bombs on wing racks. B-25Cs and B-25Ds occasionally carried a 2,000-lb (907-kg) torpedo externally for shipping attacks. Maximum speed of this version was 284 mph (457 km/h) at 15,000 ft (4,570 m).
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<p>A total of 455 B-25Cs and 40 B-25Ds was supplied to the UK, serving with Nos 98, 180, 226, 305, 320 and 342 Squadrons of the RAF based in the UK under the designation Mitchell Mk II. Some 182 B-25Cs were supplied to the Soviet Union (although eight were lost at sea in transit), as well as 688 B-25Ds. Others were delivered to Canada, mainly for use as trainers.
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<p>The XB-25E and XB-25F designations covered two trials aircraft with deicing equipment, so the next production version was the B-25G, developed from the XB-25G prototype which had been taken from the B-25C production line and modified to mount a standard US Army 75-mm field gun in the nose. The production B-25G, of which 405 were produced, carried an M4 75-mm gun with 21 15-lb (6.81-kg) shells, and was developed for anti-shipping strikes in the Pacific theater. Loaded by the navigator/bombardier by hand, the big gun could seldom fire more than four shots in a single attack. The four-crew B-25G was not regarded as successful, but a great improvement was found in the B-25H, of which 1,000 were produced with the lighter T13E1 75-mm gun and a crew of five. With four 0.5-in (12.7-mm) guns in the extreme nose, four more in blister packs on the sides of the nose, two in each of the dorsal and tail positions and two in the fuselage waist, plus a bomb load of 3,000 lb (1,361 kg) and up to eight 5-in (12.7-cm) underwing rockets, the B-25H was indeed a formidable aircraft and was highly successful in operations against the Japanese.
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<p>The main and by far the most widely-used version of the B-25 was the B-25J variant, of which 4,318 (as B-25J-NCs) were built at a new factory at Kansas City, Kansas. This reverted to a six-man crew and a glazed nose, without 75-mm gun but retaining the four 'blister' guns, and the dorsal turret was moved forward to a position immediately aft of the pilots' cockpit; power was provided by two R-2600-29 radials.
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<p>Among the groups to fly the B-25J were the 3rd, 38th, 41st, 42nd and 345th Bombardment Groups (Medium) in the Pacific theater; the 12th, 310th, 321st and 340th Bombardment Groups (Medium) in the Mediterranean theater; the 341st Bombardment Group (Medium) in Southeast Asia; and the 13th, 17th, 25th and 309th Bombardment Groups (Medium) in the USA. Further Medals of Honor were awarded to B-25 pilots, including a posthumous award to Major Raymond H. Wilkins, commanding the 8th Bomb Squadron, 3rd Bomb Group, who sank two Japanese ships during an attack on Simpson Harbor, New Britain, on 2 November 1943, but, having been crippled by flak, deliberately attracted fire from a destroyer while his squadron withdrew before he himself crashed. In an attack on a heavily defended airfield in New Guinea on 18 August 1943 Major Ralph Cheli was awarded a Medal of Honor for leading the 405th Squadron, 38th Bomb Group and, despite a severely damaged aircraft, continuing to head the attack rather than disrupt his formation before crashing into the sea.
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<hi style=hdr1>Change of Armament</hi>
<p>In 1944 an analysis of B-25 operations in the Far East disclosed that most attacks were being carried out at low level and that the bombardier was seldom needed. His station was accordingly deleted and a 'solid' nose reintroduced, first as a field modification and later in the production line. In place of the two hand-held machine guns previously fitted, a battery of eight 0.5-in (12.7-mm) guns was mounted, bringing to a total of 18 the number of guns carried by later B-25Js.
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<p>Some 295 B-25Js were purchased by the UK, but 20 of these were transferred back to the USMF in North Africa; in the RAF this version was termed the Mitchell Mk III, and served almost exclusively with the UK-based squadrons already listed. Despite the large number of B-25s built, the USMF inventory at no time exceeded 2,500, the balance being composed of large numbers supplied to other friendly air forces during the last year of the war and shortly afterwards.
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<p>The B-25 was also widely used by the USMF as a reconnaissance aircraft, early B-25s (mostly Bs) being given rudimentary camera installations in the field and issued to the 89th Reconnaissance Squadron in December 1941; others followed to the 5th Photographic Reconnaissance Group in the Mediterranean early in 1943 and the 26th Observation Group in the USA. A dedicated photographic reconnaissance version, the F-10, with a fan of three cameras for trimetrogen photography in a 'chin' fairing, was not developed until 1943; all guns were removed and fuel tanks were installed in the bomb bay.
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<p>During the latter half of the war 60 B-25Cs, B-25Ds, B-25Gs and B-25Js were stripped of all operational equipment and converted for use as trainers, being given designations in the AT (later TB) categories. After the war this conversion continued with the B-25J until more than 600 had been delivered to the USAF. Some of these went on to become TB-25Ks, TB-25Ls, TB-25Ms and TB-25Ns. Others were converted to become utility and staff transports with CB, VB and ZB designations, some serving with Strategic Air Command for communications duties from 1946. The last USAF unit to fly the B-25 in service was based at Reese Air Force Base, whose TB-25L and TB-25N pilot trainers were eventually declared obsolete in January 1959.
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<p>Finally, the third largest operator of the B-25 (after the USAAF and RAF) was the US Navy, following the policy decision reached in July 1942 to allow the US Navy to take a share of land-based bomber production. Deliveries started in January 1943; of the first 50 PBJ-1Cs (equivalent to the B-25C) delivered, US Marine Corps Bombing Squadron VMB-413 was the first to receive 20. These were followed by 152 PBJ-1Ds, one PBJ-1G, 248 PBJ-1Hs and 255 PBJ-1Ss (US Navy equivalents to the B-25D, B-25G, B-25H and B-25J respectively).
<item><hi format=bold>Performance:</hi> maximum speed 275 mph (443 km/h) at 13,000 ft (3,960 m); climb to 15,000 ft (4,570 m) in 19 minutes; service ceiling 23,800 ft (7,255 m); normal range 1,350 miles (2,173 km)
<item><hi format=bold>Weights:</hi> empty 19,975 Ib (9,061 kg); maximum take-off 36,047 lb (16,351 kg)
<item><hi format=bold>Dimensions:</hi> span 67 ft 7 in (20.60 m); length 51 ft 0 in (15.54 m); height 15 ft 9 in (4.80 m); wing area 610.0 sq ft (56.67 m<su>2</su>)
<item><hi format=bold>Armament:</hi> one 75-mm T13E1 gun with 21 shells in nose, four 0.5-in (12.7-mm) machine guns in extreme nose, four in 'blisters' on side of nose, two in dorsal turret, two in extreme tail and one in each waist position of rear fuselage, plus up to eight 5-in (12.7-mm) rocket projectiles under the outer wings and up to 3,000 lb (1,361 kg) of bombs carried internally
