From "Credits for the Destruction of Enemy Aircraft, WWII" by Wesley P. Newton, Jr., Calvin F. Senning, et al, published by the Office of Air Force History, Headquarters USAF, 1978 :
... " for the Army Air Forces to record a claim for the destruction of an enemy aircraft as a credit, the flyer had to be a member of the Army Air Forces or an allied flyer assigned or attached to an Army Air Forces unit engaged in
air-to-air- combat during the period 7 December 1941 to 14 August 1945.β (Page 7)
Also:
βAn aircraft was deemed as destroyed if it were a heavier-than-air craft, manned and which one might expect to be armed, that, as a result of
air-to-air action, crashed into the ground or water, disintegrated in the air, or was abandoned by its pilot. Credit was also given for intentional ramming of an enemy aircraft or for maneuvering in such a way as to cause the enemy plane to crash.β (Page

That being said ....... the 8th Air Force ( and ONLY the 8th AF ) DID adopt a policy of counting ground "kills" in a seperate category, but counting toward "ace" status, the stated reason being that the German airfields were so well protected by FLAK and small arms fire it was as dangerous as air-to-air combat. The general consensus now is that the real reason was when it became apparent that the Luftwaffe was not coming up to fight when bomber streams were well escorted by fighters, and the main goal of the 8th was "destruction of the Luftwaffe and air superiority" before D-Day, counting aircraft destroyed on the ground as "kills" toward ace status would coax pilots down to the ground level to shoot up the very dangerous airfields the German planes were on.
After WWII, the USAF revised kill counts, and ground kills by the 8th were "cut out" of pilot's tallies, and listed in a completely seperate total.