North American B-25 Mitchell
The North American B-25 Mitchell is an American medium bomber that was introduced in 1941 and named in honor of Brigadier General William "Billy" Mitchell, a pioneer of U.S. military aviation. Used by many Allied air forces, the B-25 served in every theater of World War II, and after the war ended, many remained in service, operating across four decades. Produced in numerous variants, nearly 10,000 B-25s were built. It was the most-produced American medium bomber and the third-most-produced American bomber overall. These included several limited models such as the F-10 reconnaissance aircraft, the AT-24 crew trainer, and the United States Marine Corps' PBJ-1 patrol bomber.
Design and development
On 11 March 1939, the US Army Air Corps issued Proposal No. 39-640 specifying a medium bomber capable of carrying a bombload over a range of at top speed in excess of. North American Aviation used its NA-40B design to develop the NA-62 proposal. More state of the art compared to the competing Martin No. 179 proposal, the North American team included easy field maintenance and repair features, and according to Avery, "It promised to be an easy airplane to fly and placed no special requirements on pilot training programs." On 20 September. the Air Corps issued North American contract No. W353-ac-13258 for 184 B-25s powered by the Wright R-2600. The plane used the NACA 23017 airfoil at the wing root changing to a NACA 4409-R at the wingtip. On 19 August 1940, Vance Breese and NAA test engineer Roy Ferren flew the first flight test, when Ferren noted a severe roll-yaw condition.Preliminary flights by the Air Corps noted the Dutch roll characteristic, accentuated by wind and gusts and demanded a fix. NAA's first nine aircraft had a constant-dihedral, the wing having a consistent upward angle from the fuselage to the wingtip. "Flattening", or changing the outer wing panels dihedral to zero degrees, was a simple solution that solved the aerodynamic problem. This gave the B-25 its gull wing configuration. The 25 February 1941 flight test confirmed the change resulted in optimum flight characteristics. The vertical tail also went through five variations before being finalized. By the time of the Attack on Pearl Harbor, 130 B-25s had been delivered.
Special variations were made to accommodate photo reconnaissance, armament, long range ferry, anti-submarine patrol, winterizing, and use in a desert environment. By February 1941, the first 24 B-25s were configured with three.30 cal guns and a single.50 cal tail gun. The B-25A had self-sealing fuel cells. The B-25B had top and bottom turrets with twin.50 cal guns each, though the tail gun was removed. By December 1941, the B-25C had additional self-sealing fuel cells outboard the wing center section. By February 1942, the first B-25D, and then in May 1943, the 75mm cannon-armed B-25G series were accepted by the Air Corps. By August 1943, the B-25H had a lighter 75mm cannon, 4 nose guns instead of 2, two waist guns, two in the tail turret, four blister gun packs, and eliminated the co-pilot after Jimmy Doolittle questioned the need. In December 1943, the B-25J was introduced, the final variant and the most produced, reincorporated the co-pilot position and included a bombardier.
NAA manufactured the greatest number of aircraft in World War II, the first time a company had produced trainers, bombers, and fighters simultaneously. It produced B-25s at both its Inglewood main plant and an additional 6,608 aircraft at its Kansas City, Kansas, plant at Fairfax Airport.
After the war, the USAF placed a contract for the TB-25L trainer in 1952. This was a modification program by Hayes of Birmingham, Alabama. Its primary role was reciprocating engine pilot training.
A development of the B-25 was the North American XB-28 Dragon, designed as a high-altitude bomber. Two prototypes were built with the second prototype, the XB-28A, evaluated as a photo-reconnaissance platform, but the aircraft did not enter production.
Flight characteristics
The B-25 was a safe and forgiving aircraft to fly. With one engine out, 60° banking turns into the dead engine were possible, and control could be easily maintained down to 145 mph. The pilot had to remember to maintain engine-out directional control at low speeds after takeoff with rudder; if this maneuver were attempted with ailerons, the aircraft could snap out of control. The tricycle landing gear made for excellent visibility while taxiing. The only significant complaint about the B-25 was its extremely noisy engines; as a result, many pilots eventually suffered from some degree of hearing loss. A Clayton S stack, introduced to quench the exhaust flame, was introduced in the B-25C series. These stacks protruded through the cowling, and though they weighed less than the replaced collector ring, they reduced aircraft speed by 9 mph due to the required aircraft fairings. According to Avery, "The increase in noise as compared to collector rings ported on the outboard side of the nacelles was a general crew complaint."Durability
The Mitchell was exceptionally sturdy and could withstand tremendous punishment. One B-25C of the 321st Bomb Group was nicknamed "Patches" because its crew chief painted all the aircraft's flak hole patches with bright yellow zinc chromate primer. By the end of the war, this aircraft had completed over 300 missions, had been belly-landed six times, and had over 400 patched holes. The airframe of "Patches" was so distorted from battle damage that straight-and-level flight required 8° of left aileron trim and 6° of right rudder, causing the aircraft to "crab" sideways across the sky.Operational history
Asia-Pacific
Most B-25s in American service were used in the war against Japan in Asia and the Pacific. The Mitchell fought from the North to the South Pacific and the Far East. These areas included the campaigns in the Aleutian Islands, Papua New Guinea, the Solomon Islands, New Britain, China, Burma, and the island hopping campaign in the Central Pacific, as well as in the Doolittle Raid. The aircraft's potential as a ground-attack aircraft emerged during the Pacific war. The jungle environment reduced the usefulness of medium-level bombing, and made low-level attack the best tactic. Using similar mast height level tactics and skip bombing, the B-25 proved itself to be a capable anti-shipping weapon and sank many enemy sea vessels. An ever-increasing number of forward firing guns made the B-25 a formidable strafing aircraft for island warfare. The Paul Gunn and Jack Fox modified strafer models with four.50 caliber guns were the B-25C1/D1, while the factory B-25J was equipped with a factory made eight gun strafer nose.In Burma, bridge busting was a primary target of the Tenth Air Force 341st Bomb Group operating B-25C and D airplanes. A glide and skip technique, called "glip" bombing, was most the effective for the Burma Bridge Busters. The 341st ranged as far as the Formosa Strait, the East China coast and French Indochina.
Middle East and Italy
The first B-25s arrived in Egypt and were carrying out independent operations by October 1942. Operations there against Axis airfields and motorized-vehicle columns supported the ground actions of the Second Battle of El Alamein. Thereafter, the aircraft took part in the rest of the campaign in North Africa, the invasion of Sicily, and the advance up Italy. In the Strait of Messina to the Aegean Sea, the B-25 conducted sea sweeps as part of the coastal air forces. 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 Douglas A-20 Havoc and Douglas A-26 Invader, allowing it to reach further into occupied Europe. The five bombardment groups – 20 squadrons – of the Ninth and Twelfth Air Forces that used the B-25 in the Mediterranean Theater of Operations were the only U.S. units to employ the B-25 in Europe.Europe
In October 1943, the Ninth Air Force 340th was transferred from the African and Mediterranean theater to England in support of the assault on Germany. In November 1944 the medium bombers eliminated the use of electric locomotives along Brenner Pass.Use as a gunship
In antishipping operations, the USAAF had an urgent need for hard-hitting aircraft, and North American responded with the B-25G. In this series, the transparent nose and bombardier/navigator position was changed for a shorter, hatched nose with two fixed.50 in machine guns and a manually loaded 75 mm M4 cannon.The B-25H series continued the development of the gunship version. NAA Inglewood produced 1000. The H had even more firepower; most replaced the M4 gun with the lighter T13E1, designed specifically for the aircraft, but 20-odd H-1 block aircraft completed by the Republic Aviation modification center at Evansville had the M4 and two-machine-gun nose armament. The 75 mm gun fired at a muzzle velocity of. Due to its slow rate of fire, relative ineffectiveness against ground targets, and the substantial recoil, the 75 mm gun was sometimes removed from both G and H models and replaced with two additional.50 in machine guns as a field modification.
The H series normally came from the factory mounting four fixed, forward-firing.50 in machine guns in the nose; four in a pair of under-cockpit conformal flank-mount gun pod packages ; two more in the manned dorsal turret, relocated forward to a position just behind the cockpit ; one each in a pair of new waist positions, introduced simultaneously with the forward-relocated dorsal turret; and lastly, a pair of guns in a new tail-gunner's position. Company promotional material bragged that the B-25H could "bring to bear 10 machine guns coming and four going, in addition to the 75 mm cannon, eight rockets, and 3,000 lb of bombs."
The H had a modified cockpit with single flight controls operated by the pilot. The co-pilot's station and controls were removed and replaced by a smaller seat used by the navigator/cannoneer, The radio operator crew position was aft of the bomb bay with access to the waist guns. Factory production totals were 405 B-25Gs and 1,000 B-25Hs, with 248 of the latter being used by the Navy as PBJ-1Hs. Elimination of the co-pilot saved weight, and moving the dorsal turret forward partially counterbalanced the waist guns and the manned rear turret.