1950 Cadillac Limousine
Alfred P. Sloan, the one time chairman of General Motors (GM) is credited with creating the concept of "planned obsolescence", producing products that consumers will want/need to replace, and in the 1940s Harley Earl, his chief designer, helped perpetuate the idea by creating new "must have" designs year after year, such as variations in tail fins.
1957 Cadillac
Lockheed P-38
The first tail fin came about in 1948 when Frank Hershey, Earl's student and protege, suggested they put one on a Cadillac. This was shortly after World War II when the images of military planes where still clear in the minds of most Americans and jet engines were replacing propeller driven commercial airplanes. It is said that Earl was thinking of the Lockheed P-38 Lightning when he came up with the idea.
Meanwhile Virgil Exner, who was an up and coming designer at Chrysler and who had a reputation for "thinking outside the box", liked the fin idea. He thought maybe it would offer some aerodynamic help to the handling of the car and even tested it in a wind tunnel, but mostly he just liked the looks. The classic 1955 Chrysler 300 was his first version
1960 Chrysler LeBaron
That started the war of the fins and by 1957 most American automakers had at least one car on the market with obvious fins sticking up or out from the rear fenders. The classic 1957 Chevrolet is a typical example but Fords, Lincolns, Thunderbirds, Studebakers, Oldsmobiles, Buicks and Pontiacs were just as obvious that year.
1959 Cadillac
The tail fin contest peaked with Earl's design of the classic 1959 Cadillac El Dorado Biarritz. It was an expensive proposition because it required a lot of hand work to form and weld the shape and while it got a lot of attention it also appeared to be excessive. From that point on most manufactures worked to find a new design to replace it and by the 1965 model year the fin fad was over.