Although the Duesenberg brothers were world-class engineers, they were neither good businessmen nor administrators; they were unable to sell all the units of their first passenger car, the Model A. This had the Duesenberg Straight-8 engine, the first "mass-produced" straight eight engine in the U.S. It was an extremely advanced and expensive automobile (prices began at $6,500), offering features such as an overhead camshaft, four-valve cylinder heads, and the first four-wheel (16") hydraulic brakes (designed by Fred in conjunction with Lockheed) offered on a passenger car anywhere (predating Adler's introduction to the European market in 1926 on the "Standard 6"). The Model A was a lighter and smaller vehicle than the competition. It was among the most powerful and the fastest cars of its time. Among the celebrities who purchased this model were Tom Mix and Rudolph Valentino.