The American Navy (more properly, the Continental Navy) started humbly on Friday, October 13, 1775, when the Second Continental Congress voted to equip two ships to intercept supplies that the British were shipping to their troops in America. The decision was recorded in the journal of the congress:
Resolved, That a swift sailing vessel, to carry ten carriage guns, and a proportionable number of swivels, with eighty men, be fitted, with all possible despatch, for a cruize of three months, and that the commander be instructed to cruize eastward, for intercepting such transports as may be laden with warlike stores and other supplies for our enemies, and for such other purposes as the Congress shall direct.
That a Committee of three be appointed to prepare an estimate of the expence, and lay the same before the Congress, and to contract with proper persons to fit out the vessel.
Resolved, That another vessel be fitted out for the same purposes, and that the said committee report their opinion of a proper vessel, and also an estimate of the expence.
Up to this point, some individual colonies had sent out ships on military missions, but these were the first ships to act under the direction of the united colonies. Some people might think it unlucky that the Navy began on Friday the 13th, but the Navy made it through the war, so I guess it was OK.