The Schneider Grunau 7 Moazagotl was a high-performance sailplane designed in Germany in 1933 specifically for fast, long distance flying using strong thermals.
The phenomenon was communicated to meteorologists at an observatory near Breslau and their director passed on the observations to the glider pilots at Grunau, who thus became the first to exploit wave lift.
Plywood skinning forward of these spars around the leading edge formed a D-box; aft, the wing, including the aileron, was fabric covered.
[1] Its fuselage was ply covered and roughly oval in cross section, with a somewhat pointed nose and tapering gently and uniformly from wing to tail.
The cockpit, just ahead of the leading edge, was enclosed by a plywood cover in Musterle fashion, providing illumination and limited views through small windows and portholes.
The sole example was preserved in the Hornberg Museum until the end of World War II, when it was deliberately burned to prevent it falling into allied hands.