There were not that many circular aircraft built before World War II. One of them was the Sacs AS-6. Developed by Mitteldeutsche Motorwerke Company. This was a polyglot plane. It was assembled with the cockpit and seat and landing gear of a wrecked ME 109. The engine was the Argus As 10C-3 240 horsepower engine cannibalized from an ME-108. Otherwise it was a ribbed plywood construction. The plane itself had a few flight tests but never proved air worthy. It did become airborne in the winter of 1944, but the only prototype was damaged in an Allied strafing attack.
sources: luft46.com, wikipedia
The US Vought Corporation did develop a similar configuration in the V-173 Flying Flapjack.