The G model of the B-17 had its first flight in the summer of 1943. If we follow air combat above Europe from September 1943 onwards, USAAF bombers were first escorted by P-47 Thunderbolts and P-38 Lightnings, before these were gradually and partially replaced by P-51 Mustangs from the early 1944.

The book Jg26: Luftwaffe Fighter Wing War Diary by Donald Caldwell details encounters between this major fighter wing and USAAF fighters escorting B-17s. While exact bomber models are not indicated, the action takes place from September 1943 onwards, so from the starting point where G models appeared above Europe.

This account is dated 2 September 1943:

Hptm. Naumann and his small force got lucky while returning from Creil. They were flying at 7400 metres (25,000 feet), in thick but broken clouds, and spotted two flights of P-47s in a clear layer below. Col. Zemke had decided to make one last turn and sweep the bombers' return course with part of his group. [page 147]

The same book also lists P-47s in Jg26 victory claims tables for the remaining months of 1943. A P-38 appears in claims for November and P-51s appear in claims starting in February 1944.