North American B-25 Mitchell B-25B

Beginning with the 65th Mitchell to leave Inglewood, the production line switched to the B-25B, company designation NA-62B. The essential difference from the B-25A was a much heavier defensive armament, dictated by the combat reports arriving from Europe: the air war there was showing that a medium bomber defended by hand-held 0.30 in guns could not hold off modern fighters. North American answered with power-operated turrets, the solution that the rest of Mitchell production would go on refining.

The most conspicuous change was a Bendix L-type power-operated turret on top of the rear fuselage, behind the wing trailing edge, carrying a pair of 0.50 in machine guns. Beneath the belly a second pair of 0.50 in guns sat in a solid, remotely controlled Bendix turret that could be retracted into the fuselage when not in use, the barrels fitting into slots in the underside. That ventral turret was the most disliked feature of the aircraft. The guns were sighted through a complex periscope of lenses and mirrors mounted on top of the turret's central column; a kneeling gunner peered into the eyepiece and directed the weapons with a dual hand control, unable to see either what his hands were doing or the barrels of his own guns. Tracking a fighter through the sight often made him dizzy, and sometimes so disoriented and nauseous that he could not fire at all. Lowering the turret and setting it up for firing took 55 seconds; if it was lowered too fast the retracting micro-switch could be damaged, jamming the turret down and creating enormous drag, and on unimproved airfields it collected mud and dust that obscured its optics. Combat units generally decided it was more trouble than it was worth and removed it in the field, saving 600 lb and freeing useful space for a long-range fuel tank.

Other changes were less obvious. The stowable 0.30 in waist gun of the B-25 and B-25A was deleted and the side hatches with their Plexiglas windows were much reduced in size. The bombardier's 0.30 in nose gun was retained. The 0.50 in tail gun, however, was eliminated, and the position that had housed it shrank to little more than a prone observation post ending in a transparent cap; the armour plate in the extreme tail also went. The crew remained five, with duties reshuffled: pilot, co-pilot, bombardier/nose gunner, navigator/upper turret gunner and radio operator/belly turret gunner. Length fell from the 54 ft 1 in of the B-25A to 52 ft 11 in.

The extra armament pushed the weight up to 20,000 lb empty and 28,460 lb loaded, with a maximum of 31,000 lb, and performance suffered again: maximum speed dropped to 300 mph at 15,000 ft, service ceiling to 23,500 ft, and the drag of the ventral turret, even retracted, cut cruising speed by 30 mph (48 km/h). The engines were still two Wright R-2600-9s of 1,700 hp for take-off and 1,500 hp at 2,400 rpm, now with Bendix Stromberg PD-13E-2 carburettors. Fuel was carried in two forward wing tanks totalling 368 gallons, two rear tanks totalling 324 gallons and the droppable 420-gallon bomb-bay tank; range remained 2,000 miles with 3,000 lb of bombs. Wingspan was 67 ft 7.7 in, height 15 ft 9 in and wing area 610 sq ft.

The first 14 B-25Bs were accepted in August 1941, and the 17th Bombardment Group was, as with the B-25, the first unit to receive them. When Japan struck Pearl Harbor on 7 December 1941 North American had delivered 130 bombers, and with completion of the last of the 120 B-25Bs in January 1942 the company fulfilled the initial contract for 184 NA-62 aircraft. Serials ran from 40-2229 to 40-2348 (40-2243 was cancelled), with company numbers 62-2898 to 62-3017. Twenty-three were delivered to the Royal Air Force as the Mitchell Mk I.

The B-25B is above all the aircraft of the Doolittle Raid, the first American air operation to strike the Japanese archipelago. In planning it, Lieutenant Colonel James Doolittle weighed the B-25B against the Martin B-26 Marauder, the Douglas B-18 Bolo and the Douglas B-23 Dragon, and the study found that the Mitchell best met the mission's requirements. Two B-25s were loaded aboard the carrier USS Hornet at Norfolk and flown off her deck without difficulty on 3 February 1942. Initial planning called for 20 aircraft, and 24 B-25Bs of the 17th Group were diverted to the Mid-Continent Airlines modification centre in Minneapolis. There they received a collapsible 160-gallon (610 L) neoprene auxiliary tank fixed to the top of the bomb bay and mounts for additional fuel cells in the bomb bay, the crawlway and the lower turret well, raising capacity from 646 to 1,141 gallons; the ventral turret was removed; the Norden bombsight was replaced by a makeshift sight nicknamed the "Mark Twain", whose materials cost 20 cents; and the liaison radio was taken out to save weight.

The 24 crews collected the modified bombers in Minneapolis and flew them to Eglin Field, Florida, from 1 March 1942, where for three weeks they trained in simulated carrier take-offs, low-level and night flying, low-altitude bombing and over-water navigation, mostly from the secluded Auxiliary Field No. 1. On 25 March the remaining 22 B-25s flew to McClellan Field, California, arriving two days later at the Sacramento Air Depot for inspection and final modifications; on 31 March, 16 of them flew on to Naval Air Station Alameda. On 1 April 1942 the 16 bombers, their five-man crews and Army maintenance personnel, 71 officers and 130 enlisted men in all, were loaded aboard Hornet, the aircraft clustered and tied down on the flight deck in launch order; the ship left San Francisco Bay at 08:48 on 2 April. Hornet's own fighters travelled stowed below decks to make room for the B-25s.

Once the force had been spotted by the Japanese, Doolittle and Hornet's captain, Marc Mitscher, decided to launch immediately, ten hours early and 170 nautical miles farther from Japan than planned. The 16 aircraft took off on 18 April 1942. Every one was lost, whether crash-landed, ditched or abandoned in the air after the crews bailed out, with a single exception: the aircraft of Captain Edward York, eighth off, serial 40-2242, which flew to the Soviet Union and landed 40 miles beyond Vladivostok at Vozdvizhenka, where it was interned and later lost in a hangar fire in the early 1950s. Of the 80 airmen, three were killed in action, eight were captured and three of those were killed in captivity by the Japanese. All 80 were awarded the Distinguished Flying Cross. Doolittle's own aircraft was 40-2344. The 17th Group, from which the Raiders had been drawn, received replacement crews and in June 1942 moved to Barksdale to convert to the B-26 Marauder.

The Doolittle Raid made the B-25B the most celebrated Mitchell, even though it was the version with the most acknowledged shortcomings, starting with its ventral turret. A B-25 rebuilt by North American to B-25B configuration from an F-10D photo-reconnaissance conversion of the B-25D, and presented to the Raiders in 1958, is displayed painted as Doolittle's 40-2344 on a reproduction of Hornet's flight deck, surrounded by figures representing Doolittle, Mitscher and the sailors and soldiers loading bombs and ammunition. Another Mitchell in Doolittle's markings is shown at the Armament Museum at Eglin, where the Raiders trained and where they held their final public reunion on 18 April 2013. The survivors' toasts were drunk from engraved silver goblets, one for each of the 80 Raiders, those of the dead turned upside down. The B-25C that followed the B-25B on the line took its turrets and corrected its shortcomings, so that the B-25B's defensive layout became the skeleton of every later Mitchell.