05/31/2010. Remarks by Johan Visschedijk: "By a happy chance, the aircraft chosen to bear the name Mitchell in memory of Colonel William "Billy" Mitchell, whose far-sighted views on the significance of air power led to his court martial in 1925, proved to be one of the most outstanding bombers used by the American Forces in WW II. Nearly 11,000 examples were built between 1940 and 1945, of which the USAAF received 9,816, and the Mitchell served on every major front during the War, with the USAAF, the USN, and air forces of other Allied Powers.

Design work began in 1938 on a medium bomber to meet an Air Corps requirement, and a prototype of this design was built by North American as a private venture. This was the NA-40-1, a three-seat shoulder-wing aircraft with a notably deep fuselage, a tricycle landing gear and twin fins and rudders. For its day, the NA-40 was heavily armed, with hand-held 0.30 in (7.62 mm) guns in the nose, dorsal and ventral positions and provision for three more fixed forward-firing guns in the wings.

Powered by two 1,100 hp Pratt & Whitney R-1830-56C3-G radial engines, the NA-40-1 was first flown in January 1939 by Paul Balfour. The engines were changed to 1,350 hp Wright GR-2600-A71s in February and, as the NA-40-2 or NA-40B the prototype was delivered to Wright Field in March. After only two weeks at Wright Field, the NA-40 was destroyed but its outstanding performance had already been noted and the USAAC requested North American to continue development, and specified a number of design changes.

The developed design, as the NA-62, had twice the bomb load of the NA-40, a wider fuselage with side-by-side seating in the cockpit, which was faired into the top line instead of projecting above it; and the wing dropped to a mid position. The crew was increased from three to five, and a tail gun station was added, with a single 0.50 in (12.7 mm) hand-held gun.

Basic design work on the NA-62 had been completed by September, 1939, and USAAC approval was indicated by the placing of a production order, that same month, for 184 B-25s. Production began at once and an airframe for static tests was completed in July 1940, followed by the first flying example which flew on August 19, 1940. The engines were 1,700 hp R-2600-9s and the gross weight had increased to 27,310 lb (13,388 kg) from the original NA-40's 19,500 lb (8,845 kg), while the wider fuselage increased the span from 66 ft (20.12 m) to 67 ft 6 in (20.57 m).

Flight tests indicated that the B-25 was deficient in directional stability, a fault which was cured by reducing the dihedral angle on the outer wing panels to give the Mitchell its distinctive gull-wing appearance. No prototypes of the
NA-62 were built as such, but the first nine B-25s were completed with the original full wing dihedral. They were followed by 15 more B-25s with the revised wing before a change was made to the B-25A (NA-62A), in which self-sealing tanks and armor protection for the pilots were introduced.

Forty B-25As were built, and in 1941 began to reach the first Mitchell operational unit, the 17th Bombardment Group (Medium), at McChord Field. Operating on anti-shipping patrols off the US West Coast at the end of 1941, a B-25A of this Group sank a Japanese submarine on December 24 in the first successful attack by this aircraft type.

The original 1939 contract was completed in 1941 with the production of 120 B-25B (NA-62B) variants, one of which (40-2243) crashed before delivery. Changes which distinguished this model of the Mitchell were concerned principally with the armament, which comprised Bendix electric dorsal and ventral turrets, with two 0.50 in (12.7 mm) guns in each, with the tail gun removed. The gross weight increased to 28,460 lb (12,909 kg). Twenty-three of the B-25Bs went to the RAF under Lend-Lease and a few also went to the Soviet AF as forerunners of the large number of later model Mitchells flown to Russia.

Some B-25Bs were also earmarked for the Netherlands AirForce in the Netherlands East Indies but these aircraft went instead to the 13th and 19th Squadrons of the 3rd Bombardment Group in Australia. In April 1942, these squadrons went into action against Japanese targets in New Guinea and the Philippines, operating, in some cases, from concealed airstrips in the Philippines themselves. Also in April 1942, 16 B-25Bs made an epic raid on Tokyo, flying off the USS Hornet 800 mls (1,287 km) away from the target.

The crews for this mission were drawn from the 17th Group and the 89th Reconnaissance Squadron, and the aircraft were modified to have 1,141 gal (4,319 l) of fuel compared with the standard 694 gal (2,627 l). The ventral turret and the Norden bombsight were removed, an auto-pilot was fitted, two wooden guns were fitted in the tail cone to discourage stern attacks, and the gross weight went up to 31,000 lb (14,061 kg). These Mitchells, led by Lt. Col. James H. Doolittle, bombed Tokyo, Kobe, Yokohama and Nagoya on April 18. All the aircraft subsequently crashed or forced-landed, but most of the crews survived, including Doolittle, who received the Congressional Medal of Honor.

Deliveries of the B-25C began before the end of 1941, and this same basic model was put into production at a second North American factory at Dallas, Texas, as the B-25D. The B-25C had 1,700 hp Wright R-2600-13 Cyclone fourteen-cylinder two-row air-cooled radial engines, an auto-pilot and external racks under the fuselage and wings. Later batches of both models had additional fuel tanks in the wing and fuselage to bring the standard capacity to 1,100 gal (4,164 l), when the gross weight became 33,500 lb (15,195 kg).

For ferrying, a 585 gal (2,214 l) tank could be fitted in the bomb bay, and the weight could be increased to 41,800 lb (18,960 kg). The external wing racks could carry eight 250 lb (113 kg) bombs to increase the total load to 5,200 lb (2,359 kg), while a 2,000 lb (907 kg) torpedo was sometimes carried externally beneath the fuselage for attacks on Japanese shipping.

Production of the B-25C totaled 1,619 at Inglewood, and of the B-25D, 2,290 at Dallas, Texas. Many of these were supplied under Lend-Lease arrangements to the RAF, Russia, Brazil, a batch intended for the Netherlands Indies were taken over by the USAAF.

During 1942, three B-25Cs were converted into prototypes of later models and were interesting as the first Mitchells to carry "X" designations. The XB-25E had experimental hot-air de-icing of the wing leading edge, while the XB-25F achieved the same results with electric elements. The third prototype was the XB-25G, in which a standard Army 2.95 in (75 mm) field gun was mounted to fire forwards through the nose."