This is a huge question that could be the subject of a book or a number of books.  Here are a few events that you should consider as important parts of this evolution.

• The French and Indian War.  This is seen as a major catalyst for the beginnings of a national identity among the colonists.
• Revolution.  For obvious reasons.
• War of 1812.  This war gave Americans a sense that theirs was a strong and relevant nation on the world stage.
• Civil War and events leading up to it.  These challenged the sense of nationalism and really led away from a sense of nationalism.
• World Wars.  These helped to move the US back towards a sense that we were all in the same boat.  They can be seen as the first major unifying events after the Civil War.

In here somewhere, you should discuss the unifying effects of technologies such as railroads, radios, movies, and other things that helped to create a national culture.