Complex sentence: The 310th Bomb Group (Medium) trained at Greenville Army Air Base from 18 September through 17 October 1942, flying North American B-25 Mitchell medium bombers.
Split into simpler sentences: The 310th Bomb Group (Medium) trained at Greenville Army Air Base from 18 September through 17 October 1942. It flew North American B-25 Mitchell medium bombers.