Under Armour just proved it's a major threat to Nike

Under Armour is stepping up its game.

CEO Kevin Plank told investors he expects the company's revenue to reach $7.5 billion by 2018. That's more than double the $3.08 billion revenue in 2014. Sales rose 29% in the second quarter

"The time has come for us to build a better house, not just a bad-ass house, which you can count on," Plank said during the meeting with investors. "I'm taking the tank with me."

Under Armour has had a successfull year, surpassing Adidas as the second-largest athletic apparel company in the United States.

Under Armour still has only a fraction of Nike's $28 billion in annual revenue, but its growth makes it a viable competitor.

The company plans to expand globally and has already signed athletes such as tennis player Andy Murray and basketball star Stephen Curry.

The brand will also appear in the upcoming Matt Damon film"The Martian."

Under Armour has slowly been eating into Nike's market share.

The brand's secret to success isn't to imitate Nike's strategies. Instead, the brand focuses on what makes it different to drive sales.

Under Armour marketing executive Adrienne Lofton told Fast Company that the brand considers itself an "underdog brand."

"We work with athletes who most people wouldn't or didn't draft in the first round, or who they wouldn't traditionally give a prima ballerina title to. We pick that athlete with a chip on their shoulder and their desire to win because it aligns with our own attitude," she told Fast Company.