Inspired by the Lockheed P-38 Lightning, GM's Harley Earl incorporated small "fins" on the rear fenders of the 1948 Cadillac models. Exner saw the design detail (also being experimented with by some Italian manufacturers) and made it his own by enlarging the fins and making them a more prominent feature. Exner believed in the aerodynamic benefits of the fins and even used wind tunnel testing at the University of Michigan — but he also liked their visual effects on the car. They were showcased on the first cars to enter the market designed under his full supervision: the 1955 Chrysler 300 series, and the Imperial. The hardtop versions of 1957 Chrysler Corporation cars also featured compound curved glass, the first to be used in a production car.