The glider Parabel (Parabola) was built by the brothers Horten in 1938. It was the most unusual of the gliders built by the brothers - the wings of the wings were not triangular like the others, but parabolic ones. Such a decision was made after testing the plane Ho.V - it was believed that the parabola would help in overcoming the problem of the "mid-effect". After a long winter storage, the plywood unit was severely damaged and had to be destroyed.
Some info and a three view :
https://cdn1.imggmi.com/uploads/2018/8/17/676a6c4f8f508797cbaecc8c268a2f83-full.jpg
http://www.airwar.ru/enc/glider/parabel.html