While not totally unique in the sense that America can claim complete origin to the following pieces, khaki trousers, penny loafers, and button down collar shirts were all popularized by Americans in and around the mid 20th century.

Khaki trousers started out as part of British military uniforms for soldiers in India. The tan color supposedly stems from soldiers dyeing their pants using a number of different substances blended together however that story is bordering on rumor. After World War 2, they became popular among the American civilian population as a comfortable alternative to wool trousers. From there, khakis and their non tan colored cousin, "chinos," took off and became the common place pants they are now.

Penny Loafers, or weejuns, were introduced to the american market by gh bass and co. in the late 20s and by the 30s and 40s were seen across ivy league campuses. By the 50s, they had taken center stage as a casual footwear in America. Through the 60s to the present they have become a staple of american fashion. They originated, however, with Norwegian peasants, and were copied after the GH Bass owner visited Norway. The name for these shoes is an ode to their country of origin.

Button down shirt collars originated with 19th century polo players in England. After visiting England and seeing these collars, John Brooks (of Brooks Brothers) brought the style back to America. The button down collar was soon paired with the oxford cloth shirt, and the backbone of American style was born. Through the 40s, 50s, and 60s, the oxford cloth buttondown, also known as the OCBD, became the single greatest representative of american fashion. The OCBD came to represent a relaxed and casual approach to clothing that was embraced by menswear enthusiasts all over the globe.

The three pieces combined, Khaki trousers, penny loafers, and the button down shirt have come to represent the classic american style.

much of my response uses Christian Chensvold's essay entitled, The Rise And Fall Of The Ivy League Look. The essay can be found here