This was the national insignia used by US aircraft throughout the inter-war period, and into the start of WWII. It was retained for the first few months of the war, but in May 1942 the design was modified by blanking out the central red circle. (US aviators rapidly found that having a red circle painted on your plane wasn't a good idea....)

These decals are scaled to represent 30", 36" and 48" diameter roundels in 1/300 scale, although they conveniently double up as twice that size in 1/600, for bombers and transports. The actual diameters are 2.5mm, 3mm and 4mm.