The imagination and ingennuity of aircraft designers never ceases to amaze me.
The advantage of this arrangement is that the propeller produces a high-speed flow, known as the propeller slipstream, through the channel generating greater lift. Custer, who was actually a relative of the ill-fated Gen. George Custer, hoped to exploit the phenomenon to develop short takeoff and landing (STOL) aircraft.

It's callked a channel wing, the idea being to get more lift by the engine placement and wing arrangement an example of this is at the MAAM in Reading Pennsylvania

You can find much more information about it at http://www.custerchannelwing.com/

That is the single most bizarre wing arangement I have yet seen.