North American B-25
Medium Bombers in Wartime
Although four-engine heavy bombers flew most of the Air Force’s bombing missions, there were many targets for which twin-engine medium bombers were more appropriate. The North American B-25 and Martin B-26 were the types used by the Air Force for attacks over shorter ranges and at lower altitudes than those usual for their larger companions.
Development of the medium bombers proceeded in parallel, both being designed to a five-place bomber specification issued by the Air Corps on January 25, 1939. Bids were opened July 5, and contracts for 184 B-25 and 201 B-26 aircraft were announced August l0, and finally approved September 20. These production contracts were made without testing a prototype, so that deliveries could be made more quickly. Both types did begin arriving at Army bases in February 1941, and both entered combat in April 1942.
North American’s B-25 design (NA-62) by Leland Atwood (1904-1999) and Raymond Rice was developed from the earlier NA-40 attack-bomber, but with a larger fuselage and fuel capacity. Two 1,700-hp Wright R-2600-9 Cyclones had Bendix carburetors and three-bladed Hamilton propellers, and the airframe was designed for easy production by component sub-assemblies.
The wing originally had unbroken dihedral from rootto tip, but to improve directional stability, the outer panels were made horizontal and the tail fins enlarged. This shape remained on all production B-25s, along with the twin rudders and tricycle gear.
The bomb bay could accommodate one 2,000, or two 1,130, or four 624-pound, or more smaller bombs, or an extra 420-gallon tank. Four hand-held flexible guns reflected 1939’s modest armament levels. A .30-caliber gun was provided in the bombardier’s nose enclosure. Two more hand-held .30-caliber guns were mounted to fire through waist windows and panels in the fuselage top and bottom. A .50-caliber gun in the tail was fired by a seated gunner.
The first B-25 was flown August 19, 1940, at Inglewood, California, by Vance Breese. It was never delivered to the Army, but was retained by the company as a test aircraft. This policy had replaced the previous custom of delivering the first aircraft of each type to Wright Field. Eventually, this aircraft became a company executive transport.
Martin’s B-26, designed by Peyton Magruder, was a more expensive and heavily-armed medium bomber. Although also a mid-wing monoplane with tricycle gear, the B-26 was highly streamlined with a circular fuselage, single rudder, and a smaller wing, despite the heavier weight. Two Pratt & Whitney 1,850-hp R-1830-5 Wasps with four-bladed Curtiss propellers, the most powerful power plants then available, were used. A double bomb bay could hold up to two 2,000, four 1,130, six 624, or 16 300-pound bombs, up to a 4,800-pound maximum.
Four .30-caliber guns had been specified for the original armament, but the war in Europe made the need for more protection obvious. Drawing on its experience with turrets for the Navy’s PBM, Martin developed an electric-power gun turret, the first such to go into American production. With built-in 3/8th-inch armor and a full 360-degree sweep, it weighed 522 pounds and was also adopted for the B-24 and other warplanes.
Two .50-caliber guns were placed in the top turret, and another was for the gunner seated in the tail. A .30-caliber gun was provided in the conical nose enclosure, and a second could be fired from an opening in the fuselage bottom behind the turret. Self-sealing fuel tanks and 553 pounds of armor were specified for the B-26A version added on option to the original contract, and by September 30, 1940, the Army decided to include these features on all B-26s under construction.
On November 25, 1940, the first B-26 was flown at Baltimore by William K. Ebel. By that time, the estimated gross weight had increased from 26,625 to over 28,700 pounds, and top speed had dropped from an estimated 323 mph to 315 mph; still 100 mph faster than the B-18As it would replace.
The bombardier’s hand-held .30-caliber gun, with 600 rounds, was retained in the nose, but Bendix power-operated turrets were provided for the remaining ships on the contract, labeled B-25B. The tail and waist guns and windows were deleted, and two .50-caliber guns with 400 rounds each were in the top turret, which added 525 pounds to the weight. Two more guns with 350 rounds each were in a retractable belly turret lowered behind the bomb bay and aimed through a periscope sight by a kneeling crewman. A 3/8-inch armor bulkhead was provided behind the gunners.
While the Bendix top turret remained throughout the war, the lower turret’s gunner found it difficult to pick up targets. Condemned as “completely hopeless” as early as October 1941, the lower turret was often deleted by operational units to save the 497-pound full weight.
The first 14 B-25Bs were accepted in August 1941, but the 15th was destroyed before delivery and deleted from the order. This was before their turrets were available, but after August all were finished with them. North American had delivered 130 bombers on this contract at the time of the Pearl Harbor attack, and 171 by the end of 1941. During January 1942, with 119 B-25Bs accepted, deliveries shifted to the new B-25C contract. By that time, the bomber had been named the Mitchell, after the most famous advocate of American air power.
The first Air Force group to use the Mitchell was the 17th Bomb Group at Pendleton, Oregon, which reached 52-plane strength (four 13-plane squadrons) in September 1941. At that time, the Air Force had ten medium bomber groups, but except for one with B-26s, the other groups had Douglas B-18 and B-23 aircraft, too obsolete for combat. When war came, these groups were assigned to off-shore anti-submarine patrols.