Clearwire, after accumulating nearly $1 billion in private financing over the past several years, is planning a $400 million initial public offering — the state's largest in six years.

The Kirkland company, founded by wireless entrepreneur Craig McCaw, is building a nationwide service that delivers broadband Internet access using a proprietary wireless technology based on WiMax. If successful, the offering will be second only to Redmond-based AT&T Wireless' $10.6 billion stock debut in 2000.

The company's intentions, disclosed in a regulatory filing Thursday, mark a milestone of sorts for an industry sometimes characterized as overhyped. The filing also answers a number of questions about the privately held company, which so far has declined to give much insight into its operations.

"It's very good news," said Ron Resnick, president of the WiMax Forum, an industry trade association. "What Clearwire has done [by filing for an IPO] is support the foundation of the argument that we are all trying to make — the world is changing and the market opportunity is ripe right now to grab business for wireless broadband."

Clearwire, founded in October 2003, provides service in 27 markets in the U.S. as well as in Belgium and Ireland. In Washington, Clearwire serves Bellingham and the Tri-Cities, and also a small test area in McCaw's Hunts Point neighborhood.

The technology is similar to cellular service; it requires towers and broadcasts a signal over licensed airwaves to a modem.

Up until now, there had only been estimates as to how many people were using the service. Maravedis, a research firm specializing in WiMax, projected that about 1 million people will be using WiMax by 2007.

Clearwire said in its filing with the Securities & Exchange Commission that it has about 88,000 U.S. subscribers and about 11,500 subscribers in Belgium and Ireland.

The filing also cleared up questions about how much spectrum the company either owns or leases. It confirmed that it has the second-largest spectrum position in the U.S., trailing Sprint Nextel. It also said, if things go according to plan, it will have enough capacity to serve up to 90 million people.

Clearwire uses a proprietary technology built by a subsidiary, NextNet Wireless of Minneapolis. It also offers voice services in some of its markets, and recently formed a partnership with Time Warner's America Online to resell Clearwire service. It also has brand-name partnerships with companies such as Intel, Bell Canada and Best Buy.

Clearwire is only the most recent billion-dollar venture McCaw has been involved with during his more-than-35-year career in wireless.

McCaw, who is Clearwire's chairman and co-chief executive with Ben Wolff, started his career by building one of the first national cellular networks — McCaw Cellular Communications. He sold that company to AT&T in 1994 for $11.5 billion, which ultimately led to the gangbuster IPO of AT&T Wireless. McCaw later rescued Nextel Communications by investing about $1.1 billion to make it one of the country's largest wireless carriers.

He has also had his failures. Teledesic went belly-up after never reaching its goal: providing a global Internet service through low-orbiting satellites. Fiber-optics network provider XO Communications, in which McCaw held a small fortune, filed for bankruptcy protection and has since reorganized.

This time, McCaw is attempting to compete with the incumbent phone and cable providers that sell DSL and cable modem service. To do so, Clearwire has already raised nearly $1 billion in equity and debt and seeks $400 million more in the public offering.

In Thursday's prospectus, the company did not say how many shares the company plans to offer or at what price. The stock would trade on the Nasdaq under the ticker symbol CLWR. It plans to use the proceeds for working capital, as well as expanding its network and acquiring more spectrum.

Clearwire will need the funds. Last year, it lost nearly $140 million on revenues of $33.5 million.

Large shareholders in the company include McCaw's investment arm Eagle River, Bell Canada, OB Wireless, the Hispanic Information and Telecommunications Network, Intel and its executives and directors.

Tricia Duryee: 206-464-3283 or tduryee@seattletimes.com