[Michael Skeet]

Revolving cannon were originally developed in the 1880s for use on capital ships as anti-torpedo boat weapons. The French were the first, I believe, and the original Hotchkiss revolving cannon was still being used in WWI, albeit not in its original form. If I recall correctly, the first aircraft-mounted cannon in French service consisted of modified single barrels taken from the three-barrel Hotchkiss.

My understanding of "flaming onions," though, is that the glowing balls were actually the 37mm tracer rounds fired to aid in aiming with this early flak gun.