As Fernando duly noted, the original Kentucky long rifles were developed during the flintlock era. The original gunsmiths responsible for their design were German immigrants who had immigrated to Pennsylvania during the early-middle 18th Century.
These gunsmiths recognized the need for a different rifle on the American frontier - a rifle that was more accurate, had a longer range, and was more efficient to use. The Kentucky long rifle met each of these requirements... the long barrel resulted in a more complete powder burn and greater muzzle velocity, which provided a flatter trajectory and greater accuracy at longer range. The reduced bore size (.36-.45 cal vs. the more predominant .50 cal of the day) meant that a frontiersman could produce more musket balls from the same volume of lead, an important advantage for the man alone on the frontier and unable to resupply. Also, the smaller shot size meant less powder was required to achieve the same velocity, again preserving precious resources that weren't readily available on the frontier.
The Kentucky long rifle also played an important role in both the Revolutionary War and the War of 1812. During the latter conflict, riflemen from Kentucky and Tennessee armed with these very same rifles played an integral role in the lopsided victory during the Battle of New Orleans, where they picked off British soldiers at will. By the time the outcome of the battle was settled, American troops lost 21 soldiers to over 2,000 casualties suffered by the Brits.
The Kentucky long rifle would continue to play an important role in early American history well into the 19th century. By the late 19th century, breech loaders and lever guns were beginning to gain popularity (and accessibility) and succeed the Kentucky long rifle as the preferred long arm on the frontier. But while the Henry Repeating Rifle and the Winchester '94 would go on to become iconic American firearms, the Kentucky long rifle was - and forever will remain - the first truly American fire arm.
Here's one such early example as noted by Fernando. This one, housed in the National Firearm Museum curated by the NRA, is unusual in possessing a double set trigger, an alteration that while more common in percussion cap variants was particularly rare among flintlock examples.
