How German 20mm Cannons Ripped Through US Bombers in WWII

A New Threat in the Skies

In the air battles over Europe, German fighter pilots faced a challenge. American B-17 Flying Fortresses and other heavy bombers could absorb fire from ordinary rifle-caliber guns and still stay aloft. Early attacks with 7.92 millimeter machine guns caused damage but rarely brought down these large aircraft. Combat footage and hard experience soon showed that a stronger weapon was needed. German engineers and pilots turned to heavier 20 millimeter cannons, and with them came a special projectile designed not just to pierce metal but to explode inside it.

The Mine Shell

Mauser developed the MG 151 autocannon and a unique high-explosive round called the “mine shell.” Unlike the solid American .50-caliber bullet, the mine shell was thin-walled and carried about 18 grams of explosive, roughly one-third of what a typical hand grenade held. Its nose fuse delayed detonation for a fraction of a second, allowing the shell to bite into an aircraft’s skin before bursting. That tiny delay meant the blast erupted within the bomber’s light aluminum frame, where control cables, hydraulic lines, and fuel tanks were most vulnerable. A single detonation could open a hole the size of a pilot’s face, spraying fragments that riddled both structure and crew.

Damage from Within

The B-17’s nickname, “Flying Fortress,” spoke to its size and multiple gun positions, not the thickness of its skin. Most of its fuselage was aluminum sheet barely thicker than a credit card. The mine shell’s purpose was to break that light structure. German camera footage showed that perhaps twenty to twenty-five direct hits could bring down a heavy bomber. Engines sometimes kept turning, but shredded oxygen lines, severed controls, and punctured fuel systems left crews helpless. U.S. Army Air Forces medical studies later found that by mid-1943, two out of five crew wounds still came from 20 millimeter fragments, even after crews began wearing flak vests that cut casualties by more than half.

Speed and Tactics

The MG 151 fired its shells at about 785 meters per second—over 2,500 feet per second—fast enough that a head-on pass ended almost instantly. Fighters such as the Messerschmitt Bf 109 could fire through the propeller hub, sending the explosive stream straight along the pilot’s line of sight. The Focke-Wulf 190, in a dedicated bomber-hunter variant, carried multiple MG 151s and sometimes extra under-wing pods, turning the aircraft into a forward-firing battery. Ground crews loaded belts of linked cartridges into these guns before each mission, knowing the destructive effect they could deliver.

Evidence in Metal and Film

Wartime photographs and gun-camera reels reveal the results. A B-24 Liberator struck by a single 20 millimeter shell shows jagged holes and a scatter of tiny exit marks where fragments tore outward. Another image captures a B-17 cockpit with its laminated windshield cracked and clouded but still intact, saving the crew from a lethal spray. Even a rugged P-47 Thunderbolt propeller bears scars where an internal burst fractured its blades. These images explain why many bombers never returned to base and therefore never appeared in official damage counts.

Shaping the Air War

Statistics from early 1944 show German fighters as the leading cause of bomber losses for the U.S. Eighth and Fifteenth Air Forces until ground artillery later claimed more. Yet those figures exclude aircraft destroyed outright in the air. Film records and wreckage tell the missing story: spirals of smoke and metal falling from altitude after a few well-placed bursts. The German tactic was simple physics. Instead of relying on countless small punctures, pilots aimed to set off explosions inside the bomber’s thin shell, letting pressure and steel fragments tear vital systems apart. Crews responded with armor, thicker windscreens, and tighter formations, but the 20 millimeter mine shell remained a constant threat, a weapon built not for deep penetration but for sudden internal destruction.