Complex sentence: Its only modern aircraft were 31 Curtiss P-40B fighters, assigned to the 20th Pursuit Squadron, which arrived in mid-May but were not operational for lack of engine coolant.
Split into simpler sentences: Its only modern aircraft were 31 Curtiss P-40B fighters, assigned to the 20th Pursuit Squadron. Although assembled in mid-May, they were not operational for lack of engine coolant.