Complex sentence: The B-29D, which had become the B-50 in December 1945, was first delivered to SAC in June 1948, and SAC's first Convair B-36 Peacemaker arrived at Kirtland AFB in September 1948.
Split into simpler sentences: The B-29D, which had become the B-50 in December 1945, was first delivered to SAC in June 1948. This was followed by SAC's first Convair B-36 Peacemaker bomber arriving at Kirtland AFB in September 1948.