The Continental Congress passed the first Flag Act on June 14, 1777.  However, the Act specified neither the proportions of the flag, the width of the stripes nor the arrangement of the stars:

"Resolved, That the Flag of the United States be made of thirteen stripes, alternate red and white that the union be thirteen stars, white in a blue field, representing a new Constellation."

A 1912 Executive Order by President Taft standardized U.S. flag proportions and a 1959 Executive Order by President Eisenhower gave us the Old Glory that is recognized around the world today.

Bonus Question:
When is Flag Day?
Answer:  A 1949 joint resolution of Congress designated June 14^th of each year as National Flag Day.