Shares of newly public Box Inc. tumbled 13 percent in after-hours trading Wednesday after the online data storage company posted a bigger loss for its fourth quarter as it ramped up spending.

Box sells online or "cloud" storage to companies and government agencies, and offers free, pared-down accounts to individual consumers. As more people use smartphones and tablets, demand has risen to store documents, photos and other digital content in remote data centers so the material can be accessed over the Internet.

But Box hasn't made a profit for a decade and is facing intense competition from much-larger rivals, including Google Inc., Microsoft Corp., and Amazon.com Inc., as well as a better-known online storage service, Dropbox. The Los Altos, California, company warned investors before going public in January that it doesn't expect to make money in the near future.

Nevertheless, Box's shares soared 66 percent in their January stock market debut, closing their first day at $23.23, with a market value of $2.7 billion. The stock has since traded as low as $16.41 but on Wednesday closed at $20.53. Shares tumbled to $17.77 in aftermarket trading following the earnings report.

Box reported a loss of $52.9 million for the three months that ended Jan. 31, compared with a loss of $43.5 million a year ago, as operating costs jumped. Losses after paying preferred dividends fell to $2.64 per share from $3.46 per share a year earlier when the company had about seven million fewer shares. Excluding one-time items, Box reported a loss of $1.65 per share for the latest quarter.

Revenue rose 61 percent to $62.6 million from $38.8 million a year earlier, however, easily topping the $58 million expected by analysts, according to FactSet. The company said it now has more than 45,000 paying customers. Billings rose 33 percent to $82 million.

"We're committed to managing the business to achieve profitability, and we have a strong balance sheet to fund this transition," co-founder and Chief Financial Officer Dylan Smith said in a statement.

Looking ahead, Box forecast first-quarter revenue of $63 million to $64 million and full-year revenue in the range of $281 million to $285 million. That tops analysts' consensus estimates of $61.9 million and $277 million, respectively.