"Chapter XII. On the way of making a second waggon which is propelled by the wind without draught animals, and which dashes violently over open country to the confusion of all troops"
Quoted from "Texaurus regis Francie" by Guido von Vigevano (1331) - see these two texts (translated to English).
The illustration shows a digital replica of Guido Vigevano's 8 metre long wind-propelled battle wagon, made by historian Ulrich Alertz (he has more drawings). For his replica, Alertz built upon both the texts and the original plans (which are hard to interpret because of the absence of perspective). He concludes that with better gearings and with the insertion of a steering mechanism the wind car would indeed have been able to speed over flat and smooth terrain - in strong winds.
Guido Vigevano's wind-propelled battle wagon was designed as a weapon in the crusades. But the wind conditions and the scarcity of flat and smooth terrain made it so that the car was never built (although the inventor probably constructed a scale model). It is remarkable that Vigevano imagined a wind car powered by a vertical windmill (hot and sophisticated technology in those days), rather than using a low-tech sail (which the Chinese had done centuries before).
Sources: 1 & 2, both in German. There is a short Wikipedia entry on Guido Vigevano in English. More "machine books" at the database machine drawings. Related: more low-tech cars / floating citadels, powered by wind and water mills.
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