Complex sentence: In September 2013, Boeing announced that it is beginning to close down C-17 production; the main wing spar of the 279th aircraft, the last aircraft, was completed in October 2014.
Split into simpler sentences: In September 2013, Boeing announced that it was beginning to close down C-17 production. The main wing spar of the 279th aircraft, the last aircraft, was completed in October 2014.