It was common practice on my last RAF Squadron (53) for groundcrew to volunteer as air gunners on the Hawker Hectors flown from 1937 until 1939. They used to wear a brass "flying bullet" on the lower left sleeve. They also received some flying pay. When the Hectors were replaced with Bristol Blenheims, this practice continued and, indeed, several volunteers with rudimentary navigational experience were press-ganged into becoming observers. I have a note which says "One of the best of these was Sgt W J Cronin who was also the Sergeant Fitter in "A" Flight.
The practice seems to have ended when the Squadron came back from France in June 1940. Then, the air gunners wore the "AG" brevet and the observers were wearing the "O" brevet (which was re-introduced in 1937). All aircrew then had the minimum rank of Sergeant which ensured that they got into a better class of prison camp if they were unlucky enough to be shot down and captured.