The XIIIa was a research glider built to test the low speed behaviour of the very advanced “b” version: a supersonic flying wing fighter in 1946 !!!. Anyway, Reimar Horten always used a glider aircraft first to test his ideas. Always resourceful, he modified a pair of his Ho III wings and mounted them with a 60º angle onto a new central section. The pilot was seated in an gondola placed, incredibly, at the apex of the trailing edge.
It was very challenging to fly, but promising enough to continue with the project.
Just look at that shape…. Non Plus Ultra.