(latin: quod erat demonstrandum "it is proven"), and later named "Conquistador del Cielo" (Spanish: "sky conqueror"), was the last in a series of racing and touring monoplane aircraft from the Granville Brothers.

A Curtiss Conqueror was the engine preferred by Cochran, but Curtiss-Wright was unable to deliver one in time, and the Pratt & Whitney Hornet originally intended for the design was substituted to make the race delivery date.

The fuselage form followed an ideal teardrop shape calculated to minimize drag and was built up from welded chromium-molybdenum alloy steel tubes with plywood formers and spruce stringers.

[5] Although most contemporaries were already moving to retractable undercarriage, the R-6 persisted with the spatted and faired units common to their previous designs, although to save time, the actual gear legs were borrowed from the Curtiss A-12 Shrike.

[7] Royal Leonard was forced down with an engine failure and had to land in Wichita, Kansas, shortly after the Gee Bee R-1/R-2 Hybrid Intestinal Fortitude disintegrated in flight, killing its pilot.

[8] In the 1936 Thompson Trophy despite most of the major contenders having dropped out before the race, Lee Miles was forced down with an engine failure on lap 11 of 15, after lagging behind Michel Détroyat's winning Caudron C.460.

He had bought the R-6H from Babb in late September 1938 and repainted it bright white with a red fuselage stripe, with the Mexican registration XB-AKM, and renamed it the Conquistador del Cielo.