Circumpolar! is a novel by Richard A. Lupoff published in 1984.
Circumpolar! is a novel in which an alternative history is depicted with vastly different geography: A highly flattened disklike "donut" topology. It describes a Great Air Race to circumnavigate the Earth by traveling through the "polar hole" (hence the title), around the "underside" of the Earth to its "rim", and back to its starting-point. The race features the von Richthofens competing against Charles Lindbergh and Howard Hughes.
Dave Langford reviewed Circumpolar! for White Dwarf #70, and stated that "It goes on a bit long and it grossly libels the Red Baron, but it's amusing."
In his Science Fact and Science Fiction: An Encyclopedia, Brian M. Stableford cited the book as one of the relatively rare examples of science fiction dealing with the Flat Earth concept.