Complex sentence: The last of the 21st PS's P-40s had been delivered 7 December and had not yet been in the air nor had their engines slow-timed, and none of its other aircraft had more than three hours of flying time.
Split into simpler sentences: The last of the 21st PS's P-40s was delivered 7 December and had not yet had their engines slow-timed. None of its other aircraft had more than three hours of flying time.