Complex sentence: On June 28,1942, RAF Polebrook was officially turned over to the USAAF and the airbase became the base of the 97th Bombardment Group, the first USAAF heavy bomber organization to arrive in the UK.
Split into simpler sentences: On June 28,1942, RAF Polebrook was officially turned over to the USAAF and the airbase became the headquarters of the 97th Bombardment Group. The 97th was the the first USAAF heavy bomber organization to arrive in the UK.