John Phillip Law was cast as Malcolm Webson, the youngest of the three skydiving barnstormers, but had to be replaced by Scott Wilson because Law injured his wrist while filming a scene. Director John Frankenheimer wanted a close-up of him landing after a parachute jump and while in harness he was raised just out of camera range by a crane, then released to float into the shot. When he landed he fell and broke his wrist badly when he put his hand out to break his fall.
Experienced but amateur skydivers, most with several thousand jumps to their credit, were brought in from California to double for the actors. During one take an unexpected gust of wind pulled the chute and caused one of the skydivers to be slammed into the ground, breaking his collarbone and dislocating his shoulder. Even though in great pain, he stayed in character and managed to get up and finish the scene. He was retained by director John Frankenheimer as a consultant.
TCM occasionally plays a 15-minute-long documentary short entitled "The Sky Divers", narrated by Wink Martindale, on the making of this film. The short is also provided on the Warner_Home_Video DVD, often cited as simply a "Behind-the-scenes documentary" featurette.
During filming at the airfield in Benton, Kansas, director John Frankenheimer wanted to get a real, horrified reaction from the extras playing the audience, so he had a mannequin dressed like a skydiver and tied it under a helicopter, which ascended several hundred feet, then released the dummy. Most of the people hadn't noticed what had been rigged up, so when it fell they thought it was a real person hitting the ground and he got the reaction he was looking for. One minor problem was that the pilot didn't gauge the wind accurately and the "skydiver" fell into some parked cars, narrowly missing some people and caving in the roof of an extra's car. The studio bought the car for several times what it was worth and the damaged vehicle spent the rest of the shooting behind one of the hangers.
The skydiving equipment the Gypsy Moths use in the film was sport parachuting state-of-the-art for the late 1960's. The three jumpers' personal gear consisted of Para-Commander main parachutes in "Piggyback" containers and harnesses made by the Pioneer Parachute Company, Pioneer jumpsuits, Bell helmets, Altimaster wrist altimeters, and French-designed and manufactured "Paraboots". The goggles they wore were a commercially-available type identical to the Polaroid M-1944 military goggle, their light gloves a commonly-available work or trucker's driving glove.
Thr parade scene featured in the movie was filmed in Abilene, Kansas and used the Abilene High School Band for the marching band. The railroad scene at the end was also filmed at the Union Pacific depot in Abilene, Kansas.
At the time the film takes place, some experienced sport parachutists used a lead fishing weight, forced over the last locking pin of their ripcord cables, to stop the cable from going all the way through the flexible steel cable housing when they opened their chutes. This technique allowed the ripcord to be pulled, opening the parachute, but the lead sinker retained the ripcord handle and cable at the end of the cable housing so the jumper wouldn't drop it. At the end of the film, when Browdy tells Malcom "When you pulled it, you really pulled it, didn't 'ya -- right out of the pack" after the memorial jump, he meant that Malcom pulled his ripcord so hard it forced the lead weight against the cable housing and right off the end of the ripcord cable -- which would take considerable strength. This practice was eventually dropped in the skydiving community when several dangling ripcord cables, retained in such a manner, got away from jumpers and wrapped around their opening parachutes, causing fatal malfunctions.