To begin this thread, a spectacular Mercedes-Benz : this 540 K was bodied in 1937 to the request of Dunlop to test the resistance of the tyres at high speeds.
It looks like an "Autobahn Kurier" but with the radiator has been deleted in favor of a more streamlined air intake.

And here is another Mercedes-Benz from the same era : a 770 K, but a very special one. Usually, the 770 Ks, (also named "Grosser Mercedes", were bodied as limousines or foor-door convertibles, and most of their clients were officials of the Nazi regime. This 770 K is the only one to have received this two-door convertible body (named "Cabriolet B" by Mercedes), built by Sindelfingen, their in-house coachbuilder, to the request of the shah of Iran.

But never delivered to him, apparently...

Another prototype from Mercedes, this time from the fifties : they studied a new range (named W 122), smaller than the ponton. Different styles were proposed, but finally, the project was abandonned.