Although the B-25G had not been as successful in combat as had been hoped, North American nevertheless received an order for 1000 cannon-carrying attack strafers under the designation B-25H.
The B-25H (NA-98) differed from the G primarily in having a much more powerful armament suite. A lighter-weight 75-mm T13E1 cannon was fitted in the nose tunnel. The nose was fitted with four fixed 0.50-inch machine guns instead of two. These guns were installed in a line across the nose and carried 400 rounds each. A pair of 0.50-inch machine guns were installed in individual blisters on the right hand side of the fuselage, each gun carrying 400 rounds. Two flexible machine guns were provided at the waist stations, installed on flexible mounts behind large cylindrically-shaped "bay windows" and firing through sockets cut into their lower rear corners. These windows were slightly staggered, with the starboard side being positioned slightly further forward of the port side so that the two gunners would not get in each other's way. Two 0.50-inch machine guns were installed in an armored Bell M-7 turret in the extreme tail. In order to accommodate a seated gunner, the aft fuselage was deepened slightly and the tail was raised seven inches further above the fuselage reference line than it was on previous Mitchell versions. A transparent cupola was provided for the tail gunner, who sat behind a wall of armor plate that protected him from gunfire from the rear. In addition, the upper turret was moved forward on the fuselage to a position above the navigator's station in order to maintain proper balance against the additional weight added to the rear by the waist guns and tail turret. This new turret was more heavily braced and had a higher dome than did previous Mitchell dorsal turrets.
One of the more controversial changes introduced by the B-25H was the deletion of the co-pilot position. The elimination of the co-pilot's seat, armor plate, and controls resulted in a saving of over 300 pounds of weight. At the position of the copilot, a jump seat for the navigator was provided since his position had now been preempted by the forward-moved dorsal turret. General James Doolittle had always questioned the need for a copilot in the B-25 and B-26. However, General George Kenney, commander of the Fifth Air Force in the Pacific, vehemently objected and claimed that he needed the second pilot for long, over-water missions under hazardous conditions. However, General Arnold overruled these objections, and the B-25H was delivered without a co-pilot's position.
B-25C-10 42-32372 was modified as the prototype B-25H. For test purposes, it was fitted with a two-gun nose and an M4 cannon as used on the B-25G, since the new lightweight T13E1 cannon was not yet available. All copilot equipment was deleted and replaced by a small jump seat for the navigator/cannoneer. The top turret was relocated from the aft fuselage forward to the navigator's compartment, where racks for 21 rounds of 75-mm ammunition were installed. The retractable ventral turret (which had generally been dead weight anyway) was finally eliminated.
The converted B-25C-10 was powered by more powerful Wright R-2600-20 engines equipped with Bendix Stromberg PR48A1 carburetors and American Bosch SF14LU-10 magnetos. Two-speed centrifugal superchargers provided a low blower ratio of 7.06 to 1 and a high ratio of 10.06 to 1. The internal diameter of the cowl nose ring was increased to 38 inches from 36 inches on previous models.
42-32372 was flown for the first time on May 15, 1943 by North American pilots Ed Virgin and Gus Pitcairn.
On production versions of the B-25H, the R-2600-20 engines gave way to the -13. The first production B-25H (serial number 43-4105) was flown for the first time on July 31, 1943 by Bob Chilton. The first B-25H was accepted by the USAAF in August of 1943.
Production B-25H aircraft carried a pair of 0.50-inch machine guns in individual blisters on the right hand side of the fuselage. On production B-25H aircraft, the crew was five--pilot, navigator/radio operator/cannoneer, flight engineer/dorsal gunner, midships gunner/camera operator, and tail gunner.
Beginning with B-25H-5 no. 43-4535, the provisions for carrying 2000-pound bombs were deleted, since such large bombs were very rarely carried in actual practice and their hoists took up a lot of space in the bomb bay.
Beginning with B-25H-5 No. 43-4405, two 0.50-inch machine guns were added in blisters on the left hand side of the forward fuselage, bringing total gun armament to fourteen. A gun sight aim pointing camera was added, and electric bomb controls were installed.
Beginning with B-25H-10 No. 43-4705, the pilot's instruments were rearranged, the size of the life-raft compartment was increased, and the brake system control cable was revised.
The first B-25Hs arrived in the FEAF in February of 1944, entering service with the 498th Squadron. The B-25H carried the extraordinarily potent armament of fourteen 0.50 inch machine guns and a 75-mm cannon, and could also carry up to 3200 pounds of bombs or a 2000-pound torpedo. The cannon carried by the B-25H was bore-sighted at 1000 yards and was generally fired at altitudes from 1000 feet at the beginning an attack to 500 feet at the end. The sighting was done with the N-6A sight, and the guns were fired by the pilot, there being no bombardier or copilot aboard. During an attack, cannon fire was usually opened at 2000 yards, with an average of three rounds being fired by the time the aircraft closed to 1000 yards.
Combat sorties confirmed that the cannon-armed B-25H offered no particular advantage over specially-adapted strafers armed exclusively with machine guns. At this stage in the war, targets specifically suited for cannon attack were relatively few and far between, and many targets that were vulnerable to the cannon were also vulnerable to a battery of 0.50-inch machine guns or to bombs. Consequently, the use of the heavy cannon was generally abandoned in the South-West Pacific by August of 1944.
In September, the B-25Hs were either passed along to the 38th Bombardment Group (which was already operating B-25Gs) or returned to depots. Unlike most other groups, the 38th Bombardment Group actually welcomed the H model.
24 B-25Hs were taken on strength by the 11th Bombardment Squadron of the Fourteenth Air Force for interdiction work in China in early 1944. They were first employed on river sweeps. In November the 11th Squadron received a few examples of the new APG-13A radar ranging equipment for use with the 75-mm cannon. This enabled the precise range to a target to be determined at all times during an attacking run, making precise aiming much simpler. This equipment was operated by the navigator, and was installed in B-25Hs 43-4584, 4071,4924, 4989, and 4601.
The last B-25H was accepted in July of 1944. In China the 341st Bombardment Group received B-25Hs in 1945, some of them serving there until the end of the war. The 1st Air Commando Group assigned to Burma was also provided with the B-25H.
43-4105/4404 North American B-25H-1 Mitchell c/n 98-21106/98-21405 43-4405/4704 North American B-25H-5 Mitchell c/n 98-21406/98-21705 43-4705/5104 North American B-25H-10 Mitchell c/n 98-21706/98-22105
Engines: Two Wright R-2600-13 Double Cyclone fourteen-cylinder air-cooled radials, rated at 1700 hp each for takeoff and 1500 hp at 2400 rpm. Equipped with Holley 1685HA carburetors. Performance: Maximum speed 275 mph at 13,000 feet. 230 mph cruising speed. An altitude of 15,000 feet could be reached in 19 minutes. Service ceiling 23,800 feet. Range 1350 miles with 3000 pounds of bombs. Weights: 19,600 pounds empty, 35,000 pounds maximum loaded. Fuel: The fuel capacity consisted of four tanks in the inner wing panels, with a total capacity of 670 US gallons. In addition, 304 US gallons of fuel could be carried in auxiliary tanks in the outboard wing panels, for a normal total fuel load of 974 US gallons. A 515-gallon tank could be installed in the bomb bay for ferrying purposes, 125 gallons of fuel could be carried in side waist positions, a 215-gallon self-sealing fuel tank installed in the bomb bay, and provisions could be made for a droppable 335-gallon metal bomb-bay fuel tank. Armament: One 75-mm T13E1 cannon in nose with 21 rounds. Four 0.50-inch machine guns in the nose with 400 rpg. Two 0.50-inch machine guns in individual blisters on the right hand side of the fuselage with 400 rpg. Beginning with B-25H-5 No. 43-4405, two 0.50-inch machine guns were added in blisters on the left hand side of the forward fuselage. Two 0.50-inch machine guns in top turret, 400 rpg. Two 0.50-inch machine guns in waist position, 200 rpg. Two 0.50-inch machine guns in tail turret, 600 rpg. Normal internal bomb load was 3000 pounds.