Although answers can vary, I would select the French and Indian War as the turning point in making English colonists into Americans. The war helped to do this in at least two ways.
First, the war helped to bring colonists together and make them think of themselves as a group. Before the war, there had been little reason for the colonists to identify themselves as Americans. The war changed this to some degree because it brought together men from the different colonies. As they gathered for a single purpose, they came to identify with people from the other colonies. They came to think that they had things in common and that they could belong to a single community.
Second, the war helped to make the colonists feel at least somewhat less positive about the British. The colonists who fought in the war saw the ways in which the British army treated its men. They disliked the brutality and contempt with which the enlisted soldiers were treated. The colonists felt that they, themselves, would never treat other men (at least not white men) like that and that they would not allow themselves to be treated in that way. This made them feel distinct from the British.
In these ways, the French and Indian War helped to create an American identity. It gave American colonists a reason to feel connected to one another and it gave them reasons to feel that they were different from the British. This was a major step in the process of making them into Americans, a process that continued as the British government took various actions that the colonists disliked in the aftermath of the war.