In 1915 - one year into World War I - bullets were too expensive to use in air-to-air combat pilot training. Camera manufacturer Thornton-Pickard had a great idea: Make a machine gun that used film - the Mark III Hythe Machine Gun Camera.
The Mark III Hythe Machine Gun Camera's design followed the Lewis Gun blueprint, a light automatic machine gun created by US Army Colonel Isaac Newton Lewis in 1911. It was first used mounted on a plane in 1912, when the US Army Captain Charles Chandler fired it from a Wright Model B Flyer. The British military liked it so much that they adopted it for the Royal Air Force's combat planes.
However, at that time there was a shortage of ammo and pilots. The latter could easily be solved by training new pilots, but this training needed a lot of the former. Thornton-Pickard got a great idea: Use a roll of film to record the trainees' manoeuvring and firing effectiveness. The idea was so good that it's still used today by Mavericks and Icemen everywhere.
The Hythe worked like the Lewis Gun: The charging handle advanced the film and cocked the shutter, while the trigger fired the shutter. The focus, aperture and shutter speed were fixed. Sadly the magazine was empty and didn't serve any function except training the pilot in changing it during the training missions. [Ignomini, Camera Classics, Wikipedia, Camerapedia via Dark Roasted Blend]