According to Frankenheimer the dancers in the go-go club were local Kansas girls, not city girls, because they looked authentic.
During the filming of Gypsy Moths at the airfield in Benton, Kansas, the 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 a couple of 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 the parked cars, narrowly missing some people and caving in the roof of an extra's ride. 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.
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 collar bone 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 the director as a consultant.
John Phillip Law was cast as Malcolm Webson, the youngest of the three skydiving barnstormers, but had to be replaced by Scott Wilson, because of an injury to his wrist. John Frankenhiemer wanted a close-up of him landing after a parachute jump and while in harness he was raised just out of camera shot 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.
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.
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.
This film is one of John Frankenheimer's two favorites of the films he has directed.