According to Forbes, Allbirds has raised $17.5 million in new investment. The Series B funding round was led by Tiger Global Management, a New York-based firm that has also backed Warby Parker, Spotify and Facebook. Elephant, Lerer Hippeau Ventures and Maveron also invested. The new infusion brings Allbirds' total funding to $27.5 million to date.

The shoe company was founded in 2015 by Tim Brown, a former New Zealand professional soccer player, and Joey Zwillinger, a biotech engineer and renewable materials expert. The startup has more than 50 employees and has been profitable since launch, according to Tim Brown. "We were absolutely diligent on every dollar we spent," he says. He declined to disclose revenue or valuation.

Allbirds released its first shoe, the Wool Runner, in March 2016. The casual sneaker is made of fine merino wool, has minimal branding and costs $95. In April, Allbirds released its second product, a slip-on sneaker called the Wool Lounger.

Consumers have flocked to the shoes for their comfort more than any other attribute, Tim Brown says. Dave Gilboa, a Warby Parker cofounder and co-CEO who's an Allbirds investor and longtime friend of Zwillinger's, counts himself among those customers. "They're more comfortable than any socks I own," he says. The shoes have a cult-like following in Silicon Valley's tech community, where everyone from Google cofounder Larry Page to venture capitalist Ben Horowitz reportedly owns a pair.

Co-CEOs Brown and Zwillinger will use the new funding for R&D on sustainable materials, which will support future products. "We're not making the materials. We're finding them," Brown says. "And our perspective is that the footwear category has gotten lazy when it comes to using natural or sustainable materials." So they're considering elements often used in categories outside of footwear, as they did with their first two wool-based shoes. Brown declined to reveal the startup's next product and says he doesn't know when it will launch.

Allbirds will also use the new funding to expand internationally and open more U.S. retail stores. It has a permanent location on the ground floor of its headquarters, on an upscale shopping street in downtown San Francisco. In July it had a pop-up store in Los Angeles' Grand Central Market. Brown thinks they may open two or more stores over the next year. New York is up next -- later this month, Allbirds will open a pop-up location in SoHo.