This being the centenary of the RAF, I became curious as to the origin of the roundel used. Apparently, in the earliest days of the First World War, when the need was recognised to put something on aircraft in order that ground troops would recognise them (and therefore not mistakenly fire on planes from their own side), the Union Flag was painted on aircraft. But there was a problem. From a distance, the St George's Cross part of the flag could be mistaken for the German Cross painted on enemy aircraft. The roundel was adopted in order to display the colours and eliminate possibility of error. Sometimes a white edge was painted around the exterior of the roundel to make it stand out against dark camouflage. In later years, when nightfighters became a factor, the white part of the roundel was done away with and only the darker red and blue parts were used.
The French also used a roundel but with the colours reversed (blue centre instead of red).
So there you go.
The French also used a roundel but with the colours reversed (blue centre instead of red).
So there you go.