He tested different fabrics, had some samples sewn up and began selling them to college teams, eventually branching out in multiple directions. This year, its17th, the company expects net revenues to exceed $2 billion.
A.B. "Buzzy" Krongard, the former CEO of Alex. Brown and one-time third-highest-ranking CIA official, considers serving as the lead director on Under Armour's board "one of the greatest experiences of my life.
"I'm blown away by him," Krongard said. "He has just an incredible intuition, just a sense of what is good. He's absolutely consumed by quality, and he's an indefatigable worker."
He met Plank after his son, a former Navy SEAL, came back from a deployment and wanted to personally thank the Under Armour chief for the big box of apparel he had sent the team. Krongard set up the meeting, and Plank sought him out for advice when he took the company public in 2005
A perennial on those best-place-to-work lists, Under Armour attracts an athletic, youthful staff. Plank himself exudes a broad-shouldered heartiness that seems fitting in either a locker room or a boardroom.
His first employee, Kip Fulks, is still on board, having risen to the rank of chief operating officer. He and Fulks, who played lacrosse at Maryland, met during their final year in college through a friend, and Plank gave him the sales pitch on his new undershirts.
Fulks remembers the euphoria of seeing the business grow, often through word of mouth among college athletes. "It was positive reinforcement 101," Fulks said.
Their own alma mater is of course an Under Armour school, as athletic apparel companies increasingly compete for all-important outfitting rights. Plank is also one of Maryland's biggest donors, writing checks and volunteering his private jet for the athletic department's needs — he's suspected as a powerful voice, justifiably or not, whenever the school changes coaches or, as it did recently, conferences.
But for Terps football coach Randy Edsall, Plank has been invaluable in raising the school's national profile. The crazy-quilt "Maryland Pride" uniforms that debuted a couple of seasons ago "created a buzz" not just in the media but among the football talent that the school would like to recruit, Edsall said.
"When you're dealing with 17- to 22-year-olds, they love that," said Edsall, who came to Maryland from a Nike school, the University of Connecticut, in 2010. "They love the Under Armour gear. They love the creativity that Under Armour brings."
Despite the suspicions, Plank isn't pulling strings behind the scenes in College Park, according to Edsall.
"It's very limited what donors can do," Edsall said of NCAA rules. "He's very proud of being a football alumnus. He's come down and talked to our team. The one thing is: Whenever you need something, he's there. He's very competitive. He wants to win at whatever he does."
As a boss, Plank is direct and demanding, Fulks said, and runs an office that is "focused and energetic.
"He always has a point of view. It's very easy to understand where you're at in the moment and what he wants you to do," he said. "I find it very refreshing. I'm very black-and-white, but it does not work for everybody."
Even as his interests veer off in other directions, Plank is not done with Under Armour. He has rebuffed takeover offers, saying no one has ever quoted a price that he thought was greater than the company's potential under his guidance.
"But you have to understand, I think our company can be really big," Plank said, "and more importantly, I think it can be really great."
Plank sees the company breaking into the Fortune 500 ranks in the next five years, restoring that particular luster to the city, which lost its last such company when Exelon took over Constellation Energy. His company's fortunes are entwined with the city's, he says.
"Telling people in these other markets, these other places, explaining to them the key to this brand and the key to what our growth will be: Frankly, it comes from Baltimore. And we're very proud of that," Plank said. "We're very proud of keeping this city on a global map."
Plank, admittedly, doesn't have much free time, but considers himself lucky to love his work. "When I get it, I'm very selfish with it," he says of the off-hours that he'll spend with his family in their Lutherville home.
"When he's home, he's home 110 percent," D.J. Plank said. The four of them like riding ATVs and bikes around the property and visiting the horses at Sagamore.
Friends say he is generous, taking a group of them recently on a trip to Richard Branson's private island in the British Virgin Islands. Plank is still getting back to form after tearing his hamstring while water skiing last summer, but hopes to run at least part of the Under Armour-sponsored Baltimore marathon this fall.
Torn muscles aside, Plank nonetheless continues to "walk with a purpose," one of the company's stated values, used in its branding and, with no spaces between the words, as the name for one of the Sagamore fillies. Where all that purposeful walking is headed, even Plank can't predict at this point, although it surely will be part of the story he plans to continue telling.
"We don't know how it ends," he says, "so we get up and we go to work every day."