We needed more than just 4 airplanes; we aimed at about 8, so I continued creating concepts. I tried to produce some variation with shapes and wing positioning. At this point I think there was still only one style of propellers. Since propellers are not that visible from above (where the in-game camera is positioned), they were not that important at this point.
From these 8 concepts, 3 made it into the first prototype (presented 2 months later at GDC 2016), 5 into the first Early Access phase and 7 into the release version. The one on the far right never made it out of concept phase and was scrapped.