Viewing WWII air force bombers formations movies:
How did the bomber gunners keep from shooting the other bombers in formation during emeny fighter attacks ?

What makes you think they did? One study that I saw, which was done shortly after the war, estimated that 40% of the damage to US bombers was friendly fire or damage from spent shell casings. The aluminum skins of the bombers was not much defense from a fired 50 cal casing when hit at a 250 MPH. Not to mention damage to props or engines. We quickly learned to put limiters on the turrets to keep gunners from shooting down their own aircraft while tracking an enemy aircraft. .

In my 50 years of collecting WWII USAAF displayed my collection at many air shows and was blessed to meet and talk to many Veterans. Many were gunners. They all mentioned the dangers of “ friendly fire”. As posted gun stops were used on top turrets to prevent gunners from shooting the tail off their A/C. Waist gunners had to be Very Careful. Formations were set up to lessen damage of “ friendly fire”.

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B-17 Bomber's Browning Machine Guns. - YouTube

The same thing happened with friendly fire between ships during combat. Gunners tracking incoming hostile aircraft would unintentionally send rounds at others ships. The same occurred in ship-to-ship combat…especially in a night action. Off Guadalcanal an American admiral was killed when his flagship was struck by shells from an American ship mistaking it for Japanese in the dark.

Let me say this about that--The Army Air Corps never participated in WWII. The Air Corps was replaced by the Army Air Force prior to the US involvement in WWII. Anyone saying otherwise is wrong--the Air Corp ceased in May 1941, prior to the bombing of Pearl Harbor. As to the OP's comments, friendly fire has been a known hazard since the beginning of organized warfare.

I certainly never flew in WWII. But have been a student of most everything WWII. I would think the waist gunners had the most opertunities to hit our planes.

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I have had my ship hit in formation by a "friendly" M-60. BTW "friendly fire" is an oxymoron. Been sprayed by shrapnel from a pair of 17 pound 2.75" rockets, etc. etc.

I would hate to be on the ground beneath a 20mm Vulcan. That's some big casings. Thank God the A-10's 30mm Avenger recycles it's empties back into the drum.

My father was a pilot on the B-17 but he never talked about his experiences.

Another problem in WWII was the shrapnel of ground-based anti-aircraft fire coming back down and damaging property on the ground and injuring or killing civilians. Same as the idiots and celebratory gunfire into the air, but on a grander scale. Millions of flak bursts, only a small fraction hitting airplanes, the rest hits the ground eventually.

Too late to ask my father-in-law. He made 51 trips in a plexiglass bubble hanging from the bottom of a B-17.