I recently purchased an M3 flak helmet used by Army Air Corps flight crews in the skies over Europe in World War Two. This is an early-generation helmet, basically just a standard infantry helmet that has been modified with hinged ear flaps to allow for the fit of headphones underneath. There is also a slight lip to allow the headphones to fit under the helmet. These helmets were used in 1944 primarily. This particular one is in fairly good shape. I put it on a standard Air Corps leather flight helmet, with standard headphones and goggles, and though the helmet is a little tight for this set-up, you get the idea.
This helmet saved the lives of flyers during the war, though it was by no means perfect. It did help against flak and shrapnel, and was usually worn in conjunction with a flak vest. Many men also put flak vests on or under their stations to protect themselves from below.