The company that made prepaid debit cards for the "unbanked" ubiquitous has a new venture: a bank. But Green Dot (GDOT) isn't planning on opening any branches. To visit this bank, you have to open up the app.

Green Dot's GoBank, announced this week in San Francisco, attempts to push mobile banking forward by making banking mobile-only. And the company seems to believe the GoBank app will delight account holders so much that they will voluntarily pay for the privilege of using it.

"Our bank was created from scratch," Green Dot founder and CEO Steve Streit said at GoBank's San Francisco unveiling. "It's not a mobile app that was bolted onto an online version of a brick-and-mortar bank."

Streit says GoBank starts from the premise that the mobile banking experience as delivered by big banks' apps feels like an afterthought. For GoBank, mobile isn't just a priority. It is the bank.

The GoBank app offers all the features and services expected of a 21st-century checking account, from paying bills to checking balances to depositing checks by smartphone camera. But Green Dot hopes GoBank's single-minded focus on the mobile user experience will peel away customers who manage most of their day-to-day lives through their phones and are tired of waiting for their current banks to catch up with that idea.

"If you look at people who have an iPhone or Android and are under 40 and are dissatisfied with their bank, it's actually quite a large market," said Sam Altman, Green Dot's vice president of mobile.

GoBank lets you check your account balance by sliding the bar on the login screen. GoBank

Altman joined Green Dot after the company last year acquired Loopt, the startup he co-founded. Loopt didn't do financial services at all; founded pre-iPhone, it was one of the first GPS-powered find-out-what-bar-your-friends-are-at apps. That part didn't matter so much to Green Dot as Loopt's years of experience in mobile usability. That skill set that comes through in GoBank's cute touches, such as "peek at your balance," which lets you check your balance simply by sliding the bar on the app's login screen. (Altman called checking balances by far the most common use for mobile banking apps.)

But the GoBank app also offers more serious conveniences not common to mobile banking. You open a GoBank account through the app. You order debit cards through the app. You reset your ATM PIN through the app. Not so long ago, all of those tasks would have required a visit to a live teller. For some, that level of inconvenience feels like an extra layer of security. But Green Dot clearly believes that a sizable population of mobile natives is out there that believes sacrificing that seamlessness just wouldn't make sense.

Green Dot says GoBank, which is FDIC-insured and will launch widely starting this spring, will make money through the standard "interchange fees" charged to merchants for debit card transactions. It will earn interest off the daily balances held by the FDIC-insured bank. It will charge fees for using out-of-network ATMs. Streit says GoBank will never charge overdraft fees.

Most surprisingly, Streit says Green Dot also expects to earn revenue off of voluntary monthly fees paid by GoBank users. GoBank will let account holders set an amount between $0 and $9, depending on what they think the service is worth. The idea might sound ludicrous. Who would give a bank any more money than they had to? More than anything, GoBank's "pay what you want" gesture seems meant to signal its confidence in the superior quality of its user experience.

"A lot of people don't love their bank," Altman said. "We tried to build an app that people would love."

The challenge for GoBank will be whether that love translates into people actually switching banks, says Jennifer Tescher, president of the Center for Financial Services Innovation. GoBank and similar competing products such as Simple, are in many ways more like modern prepaid-card accounts than traditional checking accounts – in part simply because neither offers the option of traditional printed checks, Tescher says. But it's not so much those checks that keep people from leaving. It's the hassle.

"It's one thing to think it's cool. It's another thing to get people to switch their banking relationships," Tescher says. "Banking relationships are notoriously sticky."