During World War I, allies were equipped with a sturdy and successful light artillery piece, the 75mm M1897 as it was known in the U.S. That field piece would serve as the basis for tube artillery that would be
fitted on variousWorld War II machines, from the Sherman tank to the M3
Yet when designers started looking for a way to go after shipping, from the air, that same 75mm gun would find a place in the nose of a B-25 Mitchell bomber.
One of America's best medium bombers, the B-25 was fitted with larger numbers of guns to fit some specialized roles, such as anti-shipping. The two-engined bomber would reach the summit of airborne firepower as the B-25H or, as it flown by the U.S. Marine Corps, the PBJ-1H .
Eight fixed, forward facing .50 caliber machine guns (two each in side blisters and four in the nose,) were joined by two more in a ventral turret, a pair of left and right waist guns, before concluding in a duo of tail M2s.
The PBJ also had its standard bomb racks, but it was the final piece of firepower that made the aircraft even more punchy. The 75mm M5 fired a 14 pound shell and achieved a standard muzzle velocity of around 2,000 feet per second.
Asymmetrically mounted in the PBJ-1H nose, the 75mm cannon was a lighter version of the same gun fitted into the turret of a M4 Sherman tank and possessed much of the same performance characteristics. The cannon was manually operated by the aircraft's navigator and loaded from a storage box mounted inside the fuselage. A total of 21 75mm rounds were carried on a mission.
The airborne accuracy of the 75mm was not considered excellent and its size and punch upon recoil produced problems. When the cannon was fired during a mission, crews would inspect the airframe, hydraulics and fuselage rivets upon landing. The cannon's recoil and associated vibration were known to rupture hydraulic lines and sheer rivets upon firing.
By the end of the war a number of Allied and Axis aircraft were outfitted with increasingly larger caliber guns, up to a 105mm artillery piece mounted in an Italian bomber. But when it came to an airborne tank, no one could touch the Mitchell in the sheer volume of firepower unleashed on a doomed target.