Staff Writer (Updated: 8/4/2016):
The B-25 was initially designed as a three-crew, high-mounted wing bomber utilizing twin Pratt & Whitney R-1830-S6C-3 radial engines. By the end of its production run, the system would be overhauled to include 5 to 6 crew members in various gun positions, a shoulder-mount wing assembly and twin Wright GR-2600-A71 radial engines. After over 9,800 were produced, the Mitchell had evolved into a host of variants that included trainers, dedicated ground strike, reconnaissance and torpedo carriers.
The more identifiable B-25's were a twin-engine design with the radials mounted underneath either wing element. The forward fuselage was of a greenhouse-type design affording the forward-located gunners and bombardiers a good field of vision. The pilot and co-pilot sat in a raised location just behind the nose assembly and had good vision forward, above and to the sides. A top dorsal turret was included and was mounted with twin .50 caliber machine guns. Waist gunner positions were added with single .50 caliber guns, as was a twin .50 caliber gun position in the tail. The B-25 made use of a powered-tricycle landing gear system and a decent internal bomb bay. Twin rudder assemblies differentiated the system from the other similar-looking twin-engine medium bombers of American design.
Initial B-25 designs limited the crew to just three, whereas a fuselage widening helped to increase width and seat the pilot and co-pilot side-by-side. Armor and self-sealing fuel tanks were already common in about 40 developmental models of the "A" variety. The "B" model would go on to feature 119 production aircraft with powered dorsal and ventral turret emplacements but the rear tail gunner position removed.
It would not be until the valuable and definitive "C" models joined the lines that the B-25 would come into its own. C-model construction would leap into the 1,600's and featured an autopiloting system, underwing hardpoints, provision for torpedo carrying and the identifiable forward-fixed 4 x .50 caliber machine gun pods on either side of the lower-front fuselage.
Following the C-models would be the very similar "D" series that varied in only being produced on a different production line facility. The next "G" models would feature a large-caliber 75mm cannon in the nose followed by "H" models with a similar gun system and upwards of 18 defensive machine guns positioned throughout the aircraft. Dedicated ground strike models were even featured with a devastating array of 8 x .50 caliber machine guns in a forward-fixed assembly mounted in the nose.