The design of the U.S. flag was derived from designs used for flags at the beginning of the American Revolution. On June 14, 1777, Congress declared the flag to be 13 alternating red and white stripes with a union of 13 white stars in a blue field. In 1794, after Kentucky and Vermont joined the Union, Congress added two stars and two stripes. In 1818, when five more states had joined, Congress again changed the design to 13 stripes, representing the original 13 states, and 20 stars, with a provision that an additional star be added on the admission of each new state.
Although popular tradition has invested the colors of the flag with symbolism--red for valor, white for liberty or purity, and blue for justice, loyalty, and perseverance--there is no document that historians can point to that gives this symbolism official standing.
The New England Pine Tree Flag had been in use for nearly 90 years when it was flown at the Battle of Bunker Hill in 1775.
This naval flag, used 1775 to 1776, combined two American symbols of liberty--stripes and the rattlesnake.
The Continental Colors, used by George Washington's army after Jan. 1, 1775, combined the Union Jack, the national emblem of the mother country, with the stripes of liberty.
This flag ("Betsy Ross") was authorized by Congress on June 14, 1777. The legislation did not specify how the stars should be arranged.
"That the flag of the thirteen United States be 13 stripes alternate red and white, that the union be 13 stars, white in a blue field representing a new constellation." --The First Flag Act, June 14, 1777
The 15-star, 15-stripe flag was authorized by Congress in 1795. This flag flew over Fort McHenry and became known as the Star-Spangled Banner.
The First Flag Act - June 14, 1777
The Flag History Information posted here is from The Smithsonian's Website.