It's the so-called 'Cowpens Flag', named after the Battle of Cowpens, where the flag is said to have been carried by the 3rd Maryland Regiment. After said regiment the flag is also called the '3rd Maryland flag'. The 3rd Maryland regiment was engaged in the Battle of Yorktown.

The design is nearly similar to the 'Betsy Ross flag', with the one difference, that the circle in the 'Cowpens' version consists of only 12 stars with the 13th star being in the centre.

The 'Cowpens' name might be ahistorical though, since the 3rd Maryland Regiment wasn't involved in the Battle of Cowpens (and there's no claim from any other regiment to the flag) and the first flag in Maryland wasn't constructed until 1 year after said battle. It's more likely that the flag actually dates from the War of 1812, but when Trumboll started his painting in 1819 the connection to the Revolutionary War was already established knowledge.

Leepson, Marc Flag: An American Biography