I recall reading a book about field modified variants flying out of Australia, no bombs, no bombardier, but they mounted 14 (that's right, fourteen) .50 cal M2 machine guns, all firing forward, controlled by the pilot. Sort of like a big fighter.
They were used usually at night, hunting for Japanese troop barges. They even sank a destroyer, and disintegrated one unobservant Japanese fighter who made the terminal mistake of circling around in front of one after making a pass from the rear, thinking it a normal bomber.
The nose was reportedly enveloped in one large fireball when all 14 guns were fired, and long bursts knocked off significant airspeed. Pilots said it had a substantial psychological effect on Japanese troops seeing the fireball in the sky accompanied by massive destruction on the ground.
A friend of the family was a radio operator on a 'field-modified' B25 Mitchell. His had the 75-mm nose cannon & the .50 cal blisters on the sides of the fuselage. They flew mostly out of New Guinea looking for Japanese troop ships. He said that the plane seemed to stop momentarily when the gun was fired.
Legend has it that the engineers from North American refused to build planes with the nose cannon until they saw it with their own eyes. Their calculations told them that the plane would stall, I guess.