The founder says that his story is the embodiment of the American dream.
"The thing I hate most is when people say the American dream is dead. Under Armour stands for a lot of things. One of them is the American dream," Plank tells CNBC, "and we will keep running hard and we will keep winning."
"Under Armour is a very American story and something that makes me very proud," says Plank. "And I know there is an entrepreneur out there that is looking and saying, 'Look, the guy from state school, the guy from University of Maryland, they did it! I can do it, too.'"
Plank launched the athletic apparel company in 1996. He wanted to invent a more sophisticated alternative to the cotton T-shirts he wore playing football, since those shirts got very heavy when wet with sweat.
When Plank started up 20 years ago, the Baltimore, Maryland-based brand was one of dozens of athletic apparel brands. It went public in 2005, and today it's the third-largest athletic apparel company, behind just Nike and Adidas.
It hasn't been a straight march up, however. Under Armour shares hit a peak of nearly $75 a couple years ago but recently have been hovering at less than half of that at about $32, leading Wall Street to worry whether Under Armour's recent pace of growth is sustainable.
Plank says that getting beat up sometimes is part of being in the game. But he has no plans to quit. "We are in the arena. There is blood and sweat on our face. We are fighting every single day for this brand."
"We have established this platform of not being a nice, other little player," he says. "We are one of the top three brands in the world, and we have eyes on being the No. 1 brand in the world."
The apparel company will, Plank insists, innovate its way to the top.
"We pull rabbits out of our hats for a living," Plank says, "whether it's finding Misty Copeland and the story about her, or Jordan Spieth, or Stephen Curry." Under Armour's 2014 ad campaign featuring dancer Misty Copeland was called "jaw-dropping" and dubbed "ad of the day" by AdWeek. It became a "huge viral hit" for the company, according to Time.
Plank remains as fiercely optimistic about his company as he is about his country.
"I love my family. I love Under Armour. I love America. I want America to win," says Plank. "America is a very special place. I hate seeing America in this line up of being torn down. I think we are great now. I do think there [are] opportunities to improve, but I definitely want people in America to believe."