Complex sentence: The first crude attempts at evacuating patients by air were made on biplanes in 1918, JN-4s and DH-4 bombers were soon modified by the Army Air Service for carrying litters.
Split into simpler sentences: The first crude attempts at evacuating patients by air were made on biplanes in 1918. Shortly thereafter, JN-4s and DH-4 bombers were modified by the Army Air Service for carrying litters.