The M3 and M4 steel flak helmets were introduced in 1943 and 1944 respectively, being issued to US bomber crews in response to heavy casualties suffered on daylight raids in the skies over Germany, during World War II. The M4 was designed by Colonel Malcom C Grow and replaced the M3, which was bulky to wear in the bombers gun turrets. He also designed flak suits, which were light body armour that protected the crew against fuselage metal splinters and shell shrapnels.
Used during the first bombing raids over Germany in 1943, the M3 aircrew helmet had been developed from the US M1 infantry helmet, to which hinged steel flaps were added on the sides to protect the ears and the sides of the face. It was worn mostly by gunners.
M3 aircrew helmet
Below: the M4 flight helmet