The internationalization process began in the 1930s when General Motors and Ford sent Americans to work at or manage design studios in some of their European subsidiaries. But the phenomenon I'm thinking about actually started seriously at some indefinite time around, say, 1970.
Now consider the years around 1950. Aside from Ford and GM subsidiaries and a few Detroit-influenced designs such as the Volvo PV 444 and Peugeot 203, cars tended to have a national look. That is, French cars usually seemed French, English cars English, German cars German and Italian cars Italian.
But even in those days there were hints of internationalized designs to come. This post's example is the Lancia Aurelia B56 "Florida" prototype cars of 1955 (short reference here). It was designed by the Pininfarina carozzeria, but I'm not sure if Battista "Pinin" Farina himself was the designer or if the work was done under other hands.
Although the Florida is very much Italian-looking, it has some important features that are distinctly mid-1950s American. These are (1) a wraparound windshield, and (2) having a four-door hardtop convertible body type. The term "hardtop convertible" was used in the USA for cars with conventional steel tops, but lacking a passenger greenhouse B-pillar and thus having the breezy appearance of a convertible coupe with the canvas top raised and side windows rolled down. For the 1955 model year, General Motors introduced four-door cars with the same feature, and other brands joined in as soon as they could. B-pillarless cars left the market when strong rollover-related safety regulations appeared.
The first link above mentions that four Floridas were built. Three had four doors and one was a two-door hardtop.