The question is thought provoking. I think it depends on how wide you would define art in North America. During the years noted by LeM22 aside from the descriptive drawings to document and describe events; Portraiture was the prevailing "style" of the day. John Singleton Copley, Benjamin West, and Charles Wilson Peale were painters of note during the mid 18th century. However this style was not unique to America (the colonies) one must keep in mind that many the culture and tradition of that time were emulated from where most of the colonists originated; that being England and in a much broader view,Europe where during that period portraiture was still somewhat popular. I say that because of Turner and Constable who were stretching out by painting landscapes. This too was adopted by the American artists of the time; Thomas Cole being one. This continues throughout the various movements of art I think until you arrive at Modernist art movement in the early part of the 20th century. But again I do not think the word "traditional" applies in the true sense of the word for again they were adopting and adapting a style prevalent across the pond. I feel that if you want to look for art that can truly be labled as traditional American art you must look at Folk art. Its origins are in America. It was not influenced directly by the European art scene and unlike many genres that have faded, it is ongoing.