Starwood Property Trust Inc priced shares for $20 on Tuesday and twice boosted the number of shares offered in what is likely to emerge as the largest U.S. initial public stock offering of the year.

Starwood Property, a unit of private equity firm Starwood Capital Group, sold 40.5 million shares, raising $810 million, one of the deal's underwriters said. Earlier in the day, Starwood had increased the number of shares to be sold in the IPO by 35 percent to 33.75 million.

The underwriters, led by Deutsche Bank Securities, Bank of America-Merrill Lynch and Citi, have the option to buy another 6.075 million shares. If that happens, the Starwood IPO would surpass the $828 million stock flotation in February by pediatrics nutrition maker Mead Johnson Nutrition Co, the top IPO so far in 2009.

Starwood Property, which was created in May and filed for the IPO in June, intends to invest the proceeds in commercial mortgage-backed securities sold under the U.S. government's public-private investment program, or PPIP.

The PPIP program would create a public-private partnership to take up to $1 trillion of troubled assets off bank books and unfreeze credit markets.

In its prospectus, Starwood Property said there will be a significant supply of distressed investment opportunities from sellers over the next five years.

Starwood Capital Group, controlled by Barry Sternlicht, has previously formed companies such as Starwood Hotels & Resorts Worldwide and property finance company iStar Financial Inc.

Starwood Property intends to elect and qualify to be taxed as a real estate investment trust, or REIT, for U.S. federal income tax purposes, this year.

Starwood Property's shares are set to begin trading on the New York Stock Exchange on Wednesday under the symbol STWD.

(Reporting by Phil Wahba; Editing by David Gregorio)