Key Points

• The online-only bank now has 5 million customer accounts, after passing the 4 million mark in June.
• CEO Chris Britt says Chime took four years to hit the first million customer mark but has seen a snowball effect in the past year.
• Challenger banks like Chime are attracting billions in venture capital backing. In 2019, funding for digital start-up banks has already eclipsed last year's record of $2.3 billion, according to data from CB Insights.

Branchless digital bank Chime is quickly adding to its record growth.

The San Francisco-based start-up bank, which doesn’t have any physical locations, said it passed the 5 million customer mark Wednesday. The new total comes just four months after Chime announced it had quadrupled its user base to 4 million in a single year.

Chime CEO Chris Britt said it took the company four years to hit the first million customer mark. He attributed the recent growth-spurt to a combination of zero-fee alternatives to Wall Street banks, advertising efforts and customer referrals.

“People are telling their friends about it -- that’s the best form of growth,” Britt told CNBC in a phone interview. He said Chime recently saw the best four-week period in the company’s history.

According to CB Insights, digital-first “challenger banks” are the fastest growing group among financial technology startups. In the second quarter alone, challenger banks raised $649 million across 17 venture capital deals, according to the data firm. So far this year, funding for Chime and others like Monzo and Revolut have already eclipsed last year’s record of $2.3 billion.

Chime, last valued at $1.5 billion after a Series D funding round, offers no-fee checking and saving accounts. It lets users with direct deposit get paid two days early and launched feature this summer that lets users’ balance go negative, without an overdraft fee. Despite a lack of branches, customers have access to 38,000 ATMs and access to a help number for issues that can’t be resolved through the app.

Chime and many of its digital banking peers partner with FDIC-insured banks instead of getting a banking license themselves. Its a popular set up for fintech companies who don’t have a bank charter: the banks handle the federally regulated side while start-ups focus on building apps and platforms for digitally savvy consumers. Chime works with Bancorp Bank, and earns revenue from debit card transaction fees paid by merchants.

Britt said the bulk of Chime’s customers, most of which are between the ages of 25 and 35, switch over from traditional banks. Its 5 million user total is still well below J.P. Morgan’s roughly 50 million accounts and the 37 million at Bank of America.

Chime is hardly the only alternative to traditional bank accounts. Another branchless bank Green Dot recently launched accounts with similar features, including paycheck advances. Meanwhile, successful European challenger banks Monzo, Revolut and N26 have all announced plans to launch in the U.S.

“It’s a huge market and there’s room for a number of players,” said Britt, who was an executive at Green Dot and Visa before co-founding Chime. “I’m not surprised that some people are trying to replicate the magic we’ve been able to make happen over the past year or two -- but I still think we’re very well positioned.”

-- CNBC’s Hugh Son contributed reporting.