Our flag originated from that of The East India Company, a powerful trading enterprise, whose flag happened to have 13 red and white horizontal stripes with a blue rectangle in its upper left-hand corner:
The blue rectangle bore in red and white the superimposed cross of St. Andrew and cross of St. George:
During the Boston Tea Party, the colonists dressed as Indians, boarded the East India Company's three ships, and threw overboard their entire cargoes of high-tax tea in rebellion of high British taxation (without colonist representation in British lawmaking bodies).
They also swiped the flag from the masthead of the largest ship, the Dartmouth.
When George Washington took command of the U.S. Continental Army under an elm tree in Cambridge, Massachusetts, this was the flag he used. (It was also our unofficial flag on July 4, 1776, our independence day.)
Though it was only coincidence, most of those present thought the thirteen red and white stripes represented the thirteen American colonies, but they complained about superimposed crosses in the blue rectangle that looked like a miniaturized British flag, leading to the first of many redesigns:
You can see the progression of designs for the American Flag here.
Note that the origin of the flag is disputed. Wikipedia doesn’t have a clear origin story, only suggestions. However, this account by Buckminster Fuller holds more sense than any other I’ve read.