Complex sentence: Enough construction was completed that the 376th moved into facilities on 10 September; first aircraft, Boeing B-17 Flying Fortresses, arrived later that month and were assigned to the 346th Bombardment Group.
Split into simpler sentences: Enough construction was completed that the 376th moved into facilities on 10 September. The first aircraft to arrive, Boeing B-17 Flying Fortresses, arrived later that month and were assigned to the 346th Bombardment Group.