That particular B-17 wasn't hit by AA fire. It was rammed by a Bf.109.
http://saltofamerica.com/contents/di...cle.aspx?18_90
The B-17 was famous for bringing its crews home, except for the 40% that didn't. Of 11,931 B-17s built, 4,754 didn't return.
On the other hand the B-24 was famous for
not bringing its crews home. The funny part is that out of 18,482 Liberators built, 2,112 of them didn't make it back. That's a loss rate of 11%.
http://www.taphilo.com/history/8thaf/8aflosses.shtml
None of the Allied bombers were armored.