The 677 gal (2,563 l) experimental fuelling glider XFG-1 incorporated Cornelius' experience with swept-forward wings and was intended to be towed (by a Douglas C 47) pilotless with the controls locked. After the fuel was transferred to the towplane, it was to be cast off and abandoned. An alternative use was as a piloted fuel carrier, jettisoning its undercarriage after take-off, and landing at its destination on two skids at the bottom of the fuselage.
Basically a flying fuel tank, the forward swept wings were mounted behind the cylindrical fuel tank forming the fuselage centre section. The incidence of the XFG-1 wing was adjustable on the ground at 3°, 5° and 7°. With a swept-forward wing, an increase in incidence raises the wingtips and produces dihedral, which may explain why some accounts describe the adjustment as variable-dihedral.
Two were produced for the USAAF in 1945 by the Spartan Aircraft Corporation at Tulsa, Oklahoma, under the serials 44-28059, 44-28060. Thirteen flights were made in prototype 44-28059 by Reitherman, on the last of which he was killed after failing to recover from a spin. The second machine, 44-28060, made 19 flights, but work was discontinued with the end of WW II.