There Were Several Good Medium Bombers But The B-25 Was the Best
On April 11th 1939 the North American NA-40B, NX14221, took off from Wright Field in Ohio to undertake a series of engine-out tests. During one of those tests the pilot lost control of the aircraft and it augured in. The aircraft was consumed by the subsequent fire, but the crew escaped serious injury. As a result, Douglas won the 1937 competition with their design that would become the A-20 Boston/Havoc. North American Aviation (NAA) went back to the drawing board. The lessons learned with the NA-40B were incorporated into a new twin-engine bomber design. NAA designated it the NA-62. Today we know that aircraft as the B-25 Mitchell medium bomber.
Competition With the Marauder
The NA-62 design was entered into the 1939 United States Army Air Corps (USAAC) medium bomber competition. Competing against the NA-62 was the Martin B-26 Marauder. Although there was no B-25 prototype aircraft yet, the USAAC ordered the NA-62 design into production as the B-25. The Marauder was also ordered into production without a single flyable example available for inspection, meaning both designs were essentially ordered right off the drawing board. The 9,816 B-25s produced by NAA would go on to serve with nearly every allied nation and in every theatre of World War II, train thousands of Air Force multi-engine pilots and crew members, and even star in a few movies.
Keeping The Best Out in Front
Distinctive design features of the B-25 were the twin vertical stabilizers mounted at the ends of the horizontal stabilizer in the tail of the aircraft and the pronounced “gull” wing which resulted from the outer wing panels being designed with anhedral, or downward angle, from the engine nacelles out to the wingtips. B-25s were continuously improved, modified, and upgraded throughout the design’s history, resulting in 28 distinct variants with differing primary mission capabilities, armor and armament configurations, inclusion of self-sealing fuel tanks, nose and tail gun installations, and dorsal (upper fuselage) gun turret locations. When used by the United States Navy (USN) or Marine Corps (USMC), the B-25 was designated PBJ.
Packing a Wallop
Early production variants of the B-25 were used for the famous raid on Tokyo by Doolittle and his raiders. B-25Cs were employed against Japanese targets in the Pacific beginning early in 1942 and continuing for the rest of the war. It was found that bombing from the medium altitudes at which the B-25 was designed and intended to bomb was less effective than low-level tactics. When 5th Air Force weaponeer Paul “Pappy” Gunn field-installed six forward-firing .50 caliber machine guns in their noses at the Townsville Modification Depot in Australia, and then loaded them full of parachute-retarded fragmentary bombs (parafrags), the modified 5th Air Force B-25Cs and B-25Ds became highly effective low-level strafing skip-bombers. The coming out party for these “commerce raiders” was the Battle of the Bismarck Sea. Gunn and his team removed the Bendix remote-controlled ventral turret from many of these bombers. The crews considered it useless to begin with, and after all, who needs a belly turret when you do all your flying down in the weeds anyway?
The Flying Cannon
NAA incorporated some of the field mods made by Gunn and his weaponeers into the B-25G and the B-25H models. The G had a shorter nose with two 50 caliber machine guns…and a 75 millimeter M4 cannon. The M4 was the largest cannon used on production aircraft during the war. The G model also did away with that ventral turret, added two defensive gunnery positions in the fuselage aft of the wings (the “waist” positions), and improved the tail gunner position. On the H model B-25 the M4 cannon was replaced by a lighter weight built-for-purpose cannon and more .50 caliber machine guns were added up front. The 75 millimeter cannon, due to its slow rate of fire and substantial recoil, was removed from many of the G and H models and replaced with additional .50 caliber machine guns. There was also no copilot or bombardier in the B-25H. The pilot shared space under the greenhouse with the combination navigator/cannoneer who loaded the 75 millimeter cannon. The dorsal turret was moved forward to offset the extra crew and weapon weight now carried aft in the waist and tail of the bomber. The ability to carry up to eight 5” high velocity aerial rockets (HVARs) on rails under the outer wing panels was incorporated as well.