I found the following quote at this web site http://www.acepilots.com/planes/b25.html :
The B-25A included pilot armor and self-sealing tanks. The B-25B introduced the notoriously unsuccesful Bendix ventral turret. Harold Maul, a B-25 crewman, described the ball turret in Eric Bergerud's Fire in the Sky: The Air War in the South Pacific:
"The worst thing ever designed was the bottom turret of the B-25. It was the stupidest bit of equipment. My God, the operator is sitting in one place getting a reverse image through a mirror. He couldn't hit a thing. It slowed the damn plane down, and we weren't getting belly attacks anyway. What they really needed was a tail gun, which they eventually installed."
Also:
An unusual character named Paul "Pappy" Gunn entered the B-25 story in the South Pacific, in the Fifth Air Force's Third Bomb Group. The relatively ancient Captain Gunn, 40 years old - thus the nickname "Pappy," a master of the American "can do" spirit, modified a number of aircraft for 5AF boss, General George Kenney. Gunn and his team transformed the B-25, tossing out the useless ventral ball turret, removed the bombardier position, and then added six forward-firing .50 caliber machine guns. The resulting power of these massed .50 caliber machine guns was awesome, and the 3rd BG's pilots used them to good effect, blasting away at Japanese barges and shore targets. A Zero caught by such a lead hailstorm simply exploded.
Russ 