There are no studio takes in airplanes. All close-ups of actors being airborne were done for real, sometimes with George Roy Hill, a former Marine pilot himself, flying the airplane while directing. Scenes with Robert Redford and Bo Svenson climbing out on the wing were done without any security harness or parachutes.
When Waldo is recounting his fight with Kessler at the start of the movie, he mentions that his guns jammed and Kessler, seeing that he was helpless, saluted then flew away. This actually happened to Ernst Udet (on whom Kessler is based) when flying against the French ace Georges Guynemer in 1917, only it was Udet (at that time inexperienced) who had the jam and Guynemer (a high scoring ace) who let him go.
In keeping with the aviation theme, the film opened with black-and-white Universal Pictures logo presentation in use from 1927 to 1936 which features an old early 20th Century plane flying around Earth.
The role eventually played by Bo Svenson was originally intended for Paul Newman. Newman could not be persuaded to be in the film. However, his son, Scott Newman, does appear in the film in the role of Duke.
When Axel Olsson (Bo Svenson) crashes in a water-pond after Waldo (Robert Redford) has caused his wheels to fall off, his Curtiss JN4 "Jenny" has transformed to a disguised De Havilland 82 "Tiger Moth".