This painting depicts the moment on June 28, 1776, when the first draft of the Declaration of Independence was presented to the Second Continental Congress.

This is the first completed painting of four Revolutionary-era scenes that the U.S. Congress commissioned from John Trumbull (1756–1843) in 1817. It is an enlarged version of a smaller painting (approximately 21 inches by 31 inches) that the artist had created as part of a series to document the events of the American revolution.

When Trumbull was planning the smaller painting in 1786, he decided not to attempt a wholly accurate rendering of the scene; rather, he made his goal the preservation of the images of the Nation’s founders. He excluded those for whom no authoritative image could be found or created, and he included delegates who were not in attendance at the time of the event. In all, 47 individuals (42 of the 56 signers and 5 other patriots) are depicted, all painted from life or life portraits. Some of the room’s architectural features (e.g., the number and placement of doors and windows) differ from historical fact, having been based on an inaccurate sketch that Thomas Jefferson produced from memory in Paris. Trumbull also painted more elegant furniture, covered the windows with heavy draperies rather than venetian blinds, and decorated the room’s rear wall with captured British military flags, believing that such trophies were probably displayed there.
http://www.aoc.gov/capitol-hill/historic-rotunda-paintings/declaration-independence