Fox wasn't able to rent any of he surviving WWI planes still extant, so they built their own. The aircraft were later used in "Darling Lily" and "You Can't Win Them All."
One of the stunt pilots was Peter Hillwood, who had flown Hurricanes with 56 squadron in the Battle of Britain. He was killed in an air accident in 1966.
One of the stunt pilots was Joan Hughes MBE who had been Britain's youngest female pilot at age 17. In WW2, she ferried aircraft with the Air Transport Auxiliary and was the first female flying instructor qualified to instruct on all military plane types at the time. She became one of Britian's first female test pilots.
The monoplane back story was based on the events regarding the historical Fokker D.VIII which also suffered from structural weakness and caused at least one death. The aircraft eventually entered service on the 24th October 1918.
'George Peppard' flew one of the Pfalz replicas off-screen, but did not fly in any of the scenes in the movie.