Complex sentence: TWA accepted the aircraft on 15 September 1933 with a few modifications (mainly increasing seating to 14 passengers and adding more powerful engines) and ordered 20 examples of the developed production model was called the Douglas DC-2.
Split into simpler sentences: TWA accepted the model with a few modifications (mainly increasing seating to 14 passengers and adding more powerful engines) and ordered 20 aircraft. The production model was called the Douglas DC-2.