When More Was Better: How the Mitchell Became the Most Heavily-Armed Medium Bomber of World War Two

From bombardier-led missions to 14-gun strafing monsters, the B-25‘s Pacific transformation redefined medium bomber warfare through improvisation and firepower.

The concept seemed almost absurd on paper: cramming fourteen .50-caliber machine guns into a twin-engine medium bomber originally designed for precision bombardment at altitude. Yet by the final year of World War Two, this was precisely what North American Aviation had done with the B-25 Mitchell—creating what would become the most heavily-armed production medium bomber in aviation history. The transformation from a conventional bomber to a low-level attack gunship didn’t happen in corporate boardrooms or at Wright Field test facilities. It emerged from desperation, ingenuity, and bitter combat experience in the jungles and waters of the Pacific.

The Traditional Approach Falls Short

When the first B-25 bombing mission launched on April 6, 1942, the concept was conventional enough. Mitchells of the 3rd Bombardment Group (BG), originally intended for the Royal Netherlands East Indies Army Air Force but pressed into USAAF service after Japanese victories, struck at enemy invasion forces at Gasmata on New Britain. The aircraft were standard bombardier glass-nose B-25Cs, designed for medium-altitude precision bombing—the doctrinal approach that made perfect sense in peacetime planning.

Combat reality told a different story. The dense jungle canopy of the Southwest Pacific made target identification from altitude nearly impossible. Japanese forces had become adept at dispersing and camouflaging their positions beneath the verdant cover. Shipping targets could maneuver during a bomb’s descent. The early missions highlighted an uncomfortable truth: the B-25’s original design concept wasn’t matching the demands of Pacific warfare.

The famous Doolittle Raid of April 18, 1942—when B-25Bs of the 17th BG (M) launched from USS Hornet’s deck to strike Tokyo—brought the Mitchell widespread publicity. But far from the headlines, in bases scattered across Australia and New Guinea, a more fundamental transformation was taking shape that would prove far more significant to the Pacific air war’s outcome.

Enter Pappy Gunn

The catalyst for change came from an unlikely source. Paul Irvin “Pappy” Gunn was an airline pilot and former US Navy aviator who found himself stranded in the Philippines when war erupted. While evacuating refugees from the fighting, Gunn recognized what the combat reports were revealing: attacking from altitude against elusive jungle targets and maneuvering ships simply wasn’t working. Drawing on his varied aviation background, Gunn proposed something radical—take the war to wavetop height and treetop level.

Low-level attack offered multiple advantages. Targets could be positively identified before commitment. Strafing runs could be coordinated with bombing. The element of surprise increased dramatically. But the standard B-25 wasn’t equipped for this mission. Its forward-firing armament consisted of just a single flexible .30-caliber nose gun operated by the bombardier—hardly the firepower needed to suppress ground defenses or shred lightly-armored targets.

Working alongside hard-pressed USAAF personnel, Gunn began transforming available airframes. The conversions started simply enough: fixed forward-firing .50-caliber M2 Browning machine guns were installed in the forward fuselage. The bombardier’s glass nose was painted over—no need for a bomb aimer when you’re attacking at masthead height. Then came the “package guns”—additional M2 Brownings mounted in pairs on the fuselage sides adjacent to the cockpit, up to two per side.

The bomb bay received attention, too. Standard bombs were useless at ultra-low altitude—their blast would destroy the attacking aircraft. Gunn’s team developed the “parafrag”: 60-pound fragmentation bombs fitted with small parachutes. These weapons could be delivered from a minimum altitude, the parachute slowing their descent just enough for the Mitchell to escape the fragmentation pattern. The parafrag would remain a staple of Pacific operations through the war’s end.

The 90th Bombardment Squadron (Light) of the 3rd BG pioneered these modifications in combat. The results proved immediate and dramatic. During the Battle of the Bismarck Sea in early March 1943, the converted gunship Mitchells, operating alongside other Allied aircraft, helped achieve a decisive victory over Japanese naval forces. The foundation for Pacific Mitchell operations had been laid—not by engineers at the factory, but by desperate men improvising under fire.

The Factory Takes Notice

North American Aviation’s Inglewood, California facility watched these field modifications with keen interest. If the Pacific war was demanding attack aircraft rather than conventional medium bombers, NAA could incorporate these lessons directly into the production line. The result was the B-25G, representing a dramatic departure from earlier models.

The G-model introduced “flying artillery” to medium bomber operations. A 75mm M4 cannon was installed in the lower port fuselage, firing through a large aperture in a newly-designed solid nose section. The weapon was intended primarily for anti-shipping strikes, capable of punching through merchant vessel hulls or disabling warship systems with a single hit.

But the 75mm cannon proved a mixed blessing in combat. Its slow rate of fire allowed only one or two shots during a high-speed low-level pass. Aiming the weapon accurately while managing the aircraft at wavetop height demanded exceptional skill. Some units, recognizing these limitations, deleted the cannon entirely and substituted two additional M2 machine guns in its place—initially as field conversions, later as formal air depot modifications.

The evolution continued with the B-25H. This variant retained the cannon (now the improved T13E1 75mm) but dramatically expanded the machine gun armament. Standardized package guns appeared on the fuselage sides. The defensive armament underwent revision: a new manned tail position, waist gun stations, and the repositioning of the dorsal turret to immediately behind the flight deck. Four M2 Brownings lined the solid nose. Total firepower reached fourteen .50-caliber machine guns—ten capable of firing forward during attack runs—plus the 75mm cannon.

For units that valued the cannon’s punch, this combination proved formidable. The Fifth Air Force’s 38th BG became such devotees of the weapon that they actively sought out cannon-armed examples from other units less enamored with it, integrating them alongside machine-gun-only strafers in coordinated attacks. The concept: cannon-armed Mitchells would aim for killing blows against major targets while the machine-gun ships suppressed defenses and engaged softer objectives.

The Ultimate Gunship

The B-25J represented the pinnacle of Mitchell’s combat evolution. North American eliminated the 75mm cannon entirely—operational experience suggested the massed machine gun firepower was more consistently effective—but retained all the H-model’s armament improvements. Interestingly, NAA reintroduced the glass bombardier nose for some J-models, primarily to meet requirements from bomb groups operating in the Mediterranean theater where traditional medium-altitude missions remained the norm.

But for Pacific operations, a new variant emerged: the solid-nose B-25J gunship. This version mounted eight M2 machine guns in the nose alone, arranged in two vertical rows of four. Combined with the four package guns (two per fuselage side) and the forward-rotated dorsal turret, the solid-nose J could bring fourteen .50-caliber weapons to bear in a single devastating pass.

The final enhancement came late in the war: provision for eight unguided air-to-ground rockets on zero-length mountings beneath each wing, outboard of the engines. These 5-inch High Velocity Aircraft Rockets (HVARs) added even more punch to the already formidable weapons suite.

Speed wasn’t the solid-nose J’s strong suit—approximately 290 mph at low level depending on conditions and payload—but in the Pacific’s tactical environment, firepower and maneuverability mattered more than velocity. Carrying up to 3,000 pounds of bombs internally (though loads varied widely by mission), a crew of six, and that massive armament, the J-model sacrificed some performance for devastating attack capability.

Production of the B-25J began in late 1943, with initial USAAF acceptance before year’s end. Both factory-produced solid-nose examples and field-converted variants (using modification kits) served throughout the Pacific. By early 1945, the B-25J dominated Mitchell operations in the theater, with solid-nose gunships particularly prized—though never as numerous as their glass-nose counterparts.

The Warriors and Their Machines

By the war’s final year, several numbered air forces deployed Mitchell bomb groups across the vast Pacific theater. The Fifth Air Force, arguably Mitchell’s most significant operator, had been born from desperation in Australia during February 1942 as Japanese forces swept seemingly everywhere. Led capably by the often-overlooked Lieutenant General George C. Kenney until mid-June 1944, then by Lieutenant General Ennis C. Whitehead, the Fifth fought through New Guinea’s campaigns, the costly Rabaul missions, and eventually the Philippines liberation beginning October 1944.

In early 1945, the Fifth operated two Mitchell bomb groups—the 38th and 345th BGs—plus elements of the 71st Reconnaissance Group. Both bomb groups wore flamboyant unit markings in the war’s final months. The 345th BG dubbed themselves the “Air Apaches,” with the 499th Bombardment Squadron particularly notable for impressive bat artwork adorning their aircraft’s noses and forward fuselages.

The Seventh Air Force, headquartered in Hawaii and primarily focused on Central Pacific operations, fielded the active 41st BG. The Thirteenth Air Force, formed in New Caledonia during January 1943 for South Pacific operations, operated the 42nd BG. Up in the harsh weather of the Northern Pacific, the Eleventh Air Force’s 28th BG (Composite) flew Mitchells (77th BS) alongside B-24 Liberators from bases in Alaska and the Aleutians, earning a Distinguished Unit Citation for its efforts.

The tactics pioneered by Pappy Gunn in 1942 remained central to operations three years later. Skip bombing—skipping bombs across water into ship hulls like flat stones—became a refined art. Low-level strafing runs required precise coordination and split-second timing. Parafrag attacks devastated Japanese airfields. When possible, Mitchell strikes were heavily escorted by fighters as the Allies gradually achieved air superiority.

The Philippine liberation campaign saw particularly bitter fighting. Both the 38th and 345th BGs operated from increasingly forward bases, attacking targets on land and intercepting Japanese shipping. The low-level attack profile—so successful tactically—remained hazardously dangerous, especially against concentrated anti-aircraft fire. Losses mounted, but so did the toll on Japanese forces.

Marines Join the Fight

The US Marine Corps also operated Mitchells in Pacific combat, flying navalised versions designated PBJ. North American built these variants to Navy/Marine Corps specifications, roughly equivalent to USAAF models but with notable differences. The most obvious was the completely different color scheme—Marine aircraft wore Navy tri-color camouflage rather than Army olive drab. More significantly, PBJs featured APS-3 sea search radar with distinctive wing-tip or nose-mounted antennas.

The cannon-armed PBJ-1H corresponded roughly to the USAAF’s B-25H. In November 1944, carrier compatibility trials took place aboard USS Shangri-La using PBJ-1H bureau number 35277, fitted with carrier takeoff and landing modifications. The tests proved feasibility but proceeded no further—Marine Mitchells would operate exclusively from shore bases throughout the war, sometimes alongside their USAAF counterparts, flying missions day and night.

The Final Months

As 1945 progressed, the aerial noose tightened around Japan. Mitchell units found themselves operating ever closer to the home islands. The 38th BG departed the Philippines for Okinawa in July 1945, positioning for direct strikes on Japan proper. The 345th BG relocated from Clark Field to neighboring Ie Shima island the same month. The 41st BG arrived at Okinawa in June. Only the 42nd BG remained behind, continuing mopping-up operations in the Philippines from Puerto Princesa on Palawan.

By early August, American air power over Japan was overwhelming. Mitchells contributed their full measure, striking airfields, industrial targets, harbors, and the remnants of Japanese maritime assets. Much of the merchant marine was already destroyed; many warships lay on the bottom.

Tragedy struck on August 9—the day the second atomic bomb fell on Nagasaki. Lieutenant Colonel Edwin H. Hawes, commanding officer of the 38th BG, spotted the Japanese aircraft carrier Kaiyō in Beppu Bay on Kyushu. Already damaged from previous attacks, the vessel represented a legitimate high-value target. Hawes led his Mitchells in at wavetop height. While dropping two 1,000-pound bombs, his aircraft caught in the extensive camouflage netting draped over the ship and crashed, killing all aboard. The Kaiyō received fatal damage, but the price was high.

Even as Japanese leaders pondered surrender following the atomic bombs, Pacific Mitchell units fought on. The 28th BG at Shemya in the Aleutians flew final sorties on August 13. Preparations for a possible invasion of Japan continued—nobody knew if the enemy would capitulate. The last recorded Mitchell combat operations occurred on August 14. Within days, the war finally ended.