Mountain Dew was Designed as a Whisky Chaser
In the US, Mountain Dew accounts for roughly 7% of annual soda sales. If all else is equal, that's pretty big. It was designed in 1932 with the intention of chasing moonshine whisky rather than being consumed on its own. The beer was created by two brothers who enjoyed moonshine, which was the preferred beverage in the Appalachians for discriminating drinkers.
Back then, whiskey and moonshine weren't always smooth drinks, especially when they were made by amateurs to get inebriated. The brothers wanted something to enhance their drinking experience a little smoother because at the time chasers were also very poorly made, hence Mountain Dew was conceived.
When the Hartman brothers sold Mountain Dew to the Marion, Virginia-based Tip Corporation, the modern soda was born. The previous version of Mountain Dew was a caffeine-free, lemon-lime-flavored, and clear beverage. But in order to increase sales, they gave it a facelift because it didn't look enticing to customers.
The narrative has been told in various ways. Bill Jones of Tip Corporation altered the recipe, and Bill Bridgeforth, plant manager of Tri-City Beverages, added Tri-City Lemonade to Mountain Dew to create a citrus-flavored variation.