Invasion stripes were painted on the fuselages and wings of WWII Allied aircraft, for the purpose of increased recognition by friendly forces (and thus reduced friendly-fire incidents) during and after the Normandy Landings.

Stripes were hastily applied to fighters, photo-reconnaissance aircraft, troop carriers, twin-engined medium and light bombers, but were not on four-engined heavy bombers of the U.S. Eighth Air Force or RAF Bomber Command, as there was little chance of mistaken identity. The order affected all aircraft of the Allied Expeditionary Air Force, the Air Defence of Great Britain, gliders, and support aircraft whose duties might entail their overflying Allied anti-aircraft defenses. D-Day stripes were ordered removed a month after from the upper surfaces of airplanes, and completely removed by the end of 1944.

This close-up of a Spitfire shows how hastily the stripes were painted: https://en.wikipedia.org/wiki/Invasion_stripes#/media/File:GeoffreyPageSpitfire.jpg

More Info: en.wikipedia.org