Complex sentence: The first combat unit assigned to Greenville for training was the 90th Bomb Group (Heavy), which was assigned on 21 June 1942 and flew Consolidated B-24 Liberator heavy bombers.
Split into simpler sentences: The first combat unit assigned to Greenville for training was the 90th Bomb Group (Heavy) which was assigned on 21 June 1942. It flew Consolidated B-24 Liberator heavy bombers.