The Grand Union Flag of 1775 seems to be the prototype.
A quote from
Wikipedia on the subject:
At the time of the signing of the Declaration of Independence, July 4, 1776, this was the most commonly flown flag, and was present at the naming of the country, "the United States of America," on September 9, 1776. The flag was the official flag for the beginning of the American Revolution. The Grand Union Flag formed the basis of the Stars and Stripes, which would adopt stars for independent states instead of British colours for British colonies.
Seems pretty clear to me.