Box, an Online Storage Start-Up, Is Said to File for an I.P.O. Box, an online storage and document-sharing provider, has filed confidential paperwork to go public, a person briefed on the matter said on Thursday. The company has picked Morgan Stanley, Credit Suisse and JPMorgan Chase as its top underwriters and is aiming to hold its initial public offering in the second quarter, this person added. Other specifics of the offering, including the size of the planned stock sale, have not yet been determined. The company was able to file its I.P.O. paperwork confidentially because of the Jump-start Our Business Start-ups Act, known as the JOBS Act, a law meant to make it easier for companies to go public. Among those that have used the confidential filing provision was Twitter. A spokeswoman for Box said in a statement, "We don't have anything to share at this time. We're focused on continuing to build our business and expand our customer relationships globally. Founded seven years ago, Box has become one of Silicon Valley's darlings. Executives have dropped hints that the company was seeking to go public this year, in what would be one of 2014"s most highly anticipated I.P.O."s. The company has raised about $409 million to date, including $100 million in December from a group that included Itochu Technology Ventures of Japan, Telefonica Digital and existing investors like DST Global. While a firm valuation hasn't been set yet, the start-up is expected to be valued at several billion dollars. By filing for an I.P.O., Box appears poised to beat the company to which it is often compared - Dropbox, a massive start-up most recently valued at about $10 billion - to the public markets. Unlike its peer, however, Box focuses more on corporate clients, a sector that founder Aaron Levie believes is profitable and easier to defend against newer entrants. Users of the service can receive 10 gigabytes of storage for free, or pay up to $35 a month to receive increasing amounts of data capacity. The company also provides document editing and sharing capabilities. News of Box's filing was reported earlier by Quartz.