Beginning with the 65th Mitchell, the B-25B (NA-62B) version was introduced on the production line at Inglewood. It differed from the B-25A primarily in having a much heavier defensive armament, dictated by the results of combat reports coming in from Europe.
The most readily-noticeable difference was the introduction on the B-25B of a Bendix L-type power-operated turret installed on the top of the rear fuselage which carried a pair of 0.50-inch machine guns. In addition, a pair of 0.50-inch machine guns were mounted in a solid Bendix remotely-controlled ventral turret. This turret could be retracted into the belly when not in use, the gun barrels fitting into slots in the fuselage when the turret was in the fully up position.
The retractable ventral turret was generally disliked by the Mitchell crews. The guns were sighted by a complex periscope system of lenses and mirrors mounted on the top of the turret's center column. A kneeling gunner aimed the guns by looking through the periscope sight eyepiece and directed the guns by operating a dual hand control. The optical sighting system was clumsy and cumbersome for gunners to use. Cramped in an awkward kneeling position and peering through the sight, the gunner could not see what his hands were doing, and he could not see the barrels of his weapons in his sight. The gunner often became dizzy when tracking enemy fighters through the sight, and he sometimes got so disoriented and nauseous that he could not even fire his guns. In addition, the turret took 55 seconds to be lowered into place and set up for firing. If lowered too quickly, the retracting micro switch could be damaged, jamming the turret in the down position and creating excessive aerodynamic drag. In addition, the ventral turret often collected mud and dust when operating from unimproved airfields, which obscured its sighting system. The system was usually considered more trouble than it was worth, and was generally removed by combat units in the field, saving 600 pounds in weight and providing useful space for a long-range fuel tank.
Other armament changes introduced by the B-25B were less apparent. The stowable 0.30-inch waist gun of the B-25/B-25A was eliminated, and the side hatches with their Plexiglas windows were considerably reduced in size. The B-25B retained the 0.30-inch machine gun in the extreme nose that was operated by the bombardier. However, the 0.50-inch tailgun was eliminated and the position where the tailgun had been located was greatly reduced in size, and became little more than a prone observation post, being terminated at its extreme end with a transparent cap. The armor plate in the extreme tail was removed.
The crew was five--pilot, copilot, bombardier/nose gunner, navigator/upper turret gunner, and radio operator/belly turret gunner. The length was reduced from 54 feet 1 inch on the B-25A to 52 feet 11 inches on the B-25B. The additional defensive armament of the B-25B caused the the weight to creep up to 20,000 pounds empty and 28,460 pounds fully loaded, resulting in another degradation in performance. Maximum speed was now down to 300 mph at 15,000 feet.
The first 14 B-25Bs were accepted in August of 1941. The 17th Bombardment group was the first to get the B-25B. By the time of Pearl Harbor on December 7, 1941, North American had delivered 130 bombers. With the completion of the last of 120 B-25Bs in January of 1942, North American met the initial contract for 184 NA-62 aircraft.
40-2229/2348 North American B-25B Mitchell company numbers 62-2898/62-3017 2242,2247,2249,2250,2261,2267,2268,2270,2278, 2282,2283,2292,2297,2298,2302,2344 were Doolittle Tokyo raiders. 2344 was Doolittle's plane. 2243 was cancelled.
Engines: Two Wright R-2600-9 Double Cyclone fourteen-cylinder air-cooled radials, each rated at 1700 hp for takeoff and 1500 hp at 2400 rpm. Equipped with Bendix Stromberg PD-13E-2 carburetors. Performance: Maximum Speed 300 mph at 15,000 feet. Service ceiling 23,500 feet, range 2000 miles with 3000 pounds of bombs. Weights: 20,000 pounds empty, 28,460 pounds loaded, 31,000 pounds maximum. Dimensions: Wingspan 67 feet 7.7 inches, length 53 feet 0 inches, height 15 feet 9 inches, wing area 610 square feet. Fuel: two forward wing tanks, total 368 gallons. Two rear wing tanks total 324 gallons. One droppable bomb bay tank, 420 gallons. Armament: One 0.30-inch machine gun in flexible mount in the nose. Two 0.50-inch machine guns in Bendix dorsal turret. Two 0.50-inch machine guns in retractable Bendix ventral turret.