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.
One of the stunts the movie recounts is the first outside loop. It's the stunt that kills Ezra Stiles. The first person in the U.S. to successfully perform an outside loop was Jimmy Doolittle, who went on to become a hero of WWII.
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.
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".
When Axel Olsson (Bo Svenson) crashes in a water-pond after Waldo (Robert Redford) has caused his wheels to fall off, small roller skate type wheels can be seen on the axle, which allowed the stunt plane to land back on land if it was necessary and the stunt couldn't be done.
In his book "Adventures in the Screen Trade" writer William Goldman wrote that his original idea for the opening scene was cut. The idea was to open with a young boy trying to impress a young girl by holding out his arms and actually being able to fly. We then see the sky filled with young boys flying and young girls being impressed. The director felt the tone of that scene did not match the rest of the film.
After Waldos friend Ezra Stiles (Edward Herrmann) has crashed, Waldo takes off in a Curtiss JN4 "Jenny", loses power and crashes. When the "Jenny" crashes, it has transformed itself into a disguised De Havilland 82 "Tiger Moth".
Some movie posters for this film featured a long text preamble that read: "The Second Greatest Flyer in the World. The war was over - and the world's greatest flyers had never met in combat. But Waldo was going to change all that - even if it killed him".
The name of the activity of walking on a biplane's wings whilst flying was called "wing walking" whilst the general name for the activity of performing flying tricks on airplanes flown by stunt pilots either individually or in groups was called "barnstorming." Organisations that ran these circus shows were called "flying circuses." Stunt pilots who performed these flying stunts and tricks were called "barnstormers."