One of the best studies of friendly fire I know of is:
http://usacac.army.mil/cac2/cgsc/car...bs/shrader.pdf
Rather than Fratricide, Maj. Shrader used Amicide. He provides historical models showing that typically friendly fire accounted for -2% of casualties in ground ops but could reach 30+ % in bombing/strafing friendlies.
There have been numerous "Blue on Blue" incidents in air combat, perhaps most notably when von Richthofen probably was WIA by a wingman in July '17. We can only guess at what friendly fire must've been like in Bomber Command and the Nachtjagd...