I know that US naval aviators were taught deflection shooting prior to 1939 - although I don't know from what year it started. In that the USN was unique.
It was not taught in the Luftwaffe, RAF, USAAF, Regia Aeronautica or JAAF/JNAF in 1939.
My question is was it adopted officially by any of the above air forces? If so, does anyone know what year it was?