In the mass of athletic to street sneakers, the nondescript gray trainers, at first glance, could be regarded as utilitarian and entirely forgettable.

The muted shoes belied the vibrant story of business partners Joey Zwillinger and Tim Brown, who started their successful sustainable apparel company, Allbirds, in 2014, four years after Zwillinger graduated from the Wharton School at the University of Pennsylvania and two years after Brown, a professional soccer player in his native New Zealand, retired from the sport.

Last fall, with visions of broadening its product line beyond shoes and a limited assortment of clothes, Allbirds secured $100 million in Series E funding backed by the investment firm Franklin Templeton. And in the years before that, it had captured the attention of Silicon Valley’s techies and counted Leonardo DiCaprio and Starbucks CEO Howard Schultz among its investors.

As business continued to grow for the company best known for its sleek, minimally embellished trainers made from merino wool that originally sold only online, Allbirds brought its first store to 2,090 square feet at 1709 Walnut St. in Philadelphia during the middle of the pandemic last year and hired 14 employees.

And this year, as droves of retailers have trimmed offerings at their brick-and-mortar locations or closed them altogether, Allbirds plans to expand its retail presence by adding to its list of 23 stores that span Auckland, New Zealand, to San Diego.

“In terms of why we chose to come to Philly, we already had a significant e-commerce customer base in the Philadelphia area and felt that there would be healthy demand for an in-person shopping experience,” said Travis Boyce, Allbirds’ head of global retail operations. “The Center City area has a great mix of shops and is a destination for a wide array of consumers, so it immediately came to mind as the perfect location for our first store in the city.”

Boyce did not elaborate on Allbirds’ plans for expansion.

As brick-and-mortar shopping flailed much of last year — with Philadelphia’s highbrow shopping district on Chestnut and Walnut Streets no exception — five retailers managed to open anyway. In addition to Allbirds, Atmos, Interior Define, Kate Spade New York Outlet, and Kevin O’Brien Studio introduced themselves to Center City in 2020, and Rolex is expected in March, according to the Center City District, which focuses on economic development in the city’s downtown.

Boyce declined to provide sales figures for the company, which is not publicly listed, though investors bought a stake for $1.4 billion in 2018, according to the Wall Street Journal.

Allbirds’ presence on Walnut Street adds to the mix of national retailers in Center City, which totaled 255 in 2020, out of 1,001, according to the Center City District. That constituted around 34% of the retailers in the city’s prime downtown shopping area, with the remainder being made up of independent or local stores. The majority of the retailers — 225 of them — focus on selling apparel, with the next-largest group — 138 retailers — selling jewelry and watches.

“It was well underway prior to [COVID],” Steven Gartner, executive vice president of global retail services for CBRE, said of Allbirds’ opening in Philadelphia. “But it’s certainly a great indication for both Walnut [Street], Philadelphia in general, and the growing strength of the Allbirds brand. They could have just as easily delayed or canceled opening, but they did not.”

Part of the strength of Allbirds, and what has netted its loyalists who range from Bay Area tech entrepreneurs to DiCaprio to New Jersey Gov. Phil Murphy, is what it starkly refuses to be: flashy, impractical, or garishly imprinted with logos.

It touts environmental sustainability as one of its main selling points. It says its merino wool is ethically sourced, sneaker midsoles are made from carbon-negative Brazilian sugarcane, and shoelaces spun from recycled plastic bottles.

“During the pandemic we’re seeing that our sustainable commitments are just as if not even more important to shoppers than ever,” Boyce said. “It’s a testament that people care about brand mission and want to put their purchasing power to good use. They want to do something that’s better for the world.”