During the First World War, the state of art in camouflage – for aircraft that weren’t painted in dramatically visible patterns – was mottled painting to reduce the visibility of the plane against ground or sky. But, as I found out from Nieuport 11/16 Bébé vs. Fokker Eindecker (part of Osprey’s Duel series), the Germans did at one point make an outrageous experiment to make their planes entirely disappear.
In 1916, the German air force replaced the doped fabric skin of a range of aircraft, including the Fokker E III fighter, with an early plastic, the celluloid-like Cellon. Developed by chemist Arthur Eichgrün, Cellon was used by both sides as a varnish that could be used to dope aircraft skin. It was also available as transparent solid panels, which the Germans used in this case as a replacement doped fabric. The idea was that the Cellon airplane would become invisible in the sky. The reality was less successful. Though effectively transparent in clear skies, in cloudy weather the Fokker monoplane covered in Cellon was about as visible as normal fabric. Worse yet, in bright light the plane was sufficiently reflective that it was not only noticeable but blinding to its own pilot and observer. With such unfavorable results, the project soon did what its experimental airplane had failed to do: it vanished.