Looking for a story from World War II?

I'm looking for an account of a story I heard about WWII, in which aircraft designers were trying to figure out exactly where to put armor on aircraft -- they didn't have the planes that were shot down to examine, so they had to work with the planes that were hit by enemy fire but not shot down. They eventually figured out that they had to armor the places where the surviving planes had NOT been hit. I'm looking for an accounting of the basic facts of this story (and to see if it's really true).