A $3 billion Canadian real estate firm plans to begin work shortly on what may be Prince George’s County’s largest development since National Harbor, and it is pulling out all the stops to get the FBI to build a headquarters there as a focal point. The Walton Group, one of North America’s largest land developers, purchased the 479-acre Westphalia Town Center project along Pennsylvania Avenue near Andrews Air Force Base in February of last year, and says it will begin construction this month. The first phase of the project calls for 347 town homes, more than 400 apartments, 450,000 square feet of retail and a 150-room hotel. Under previous ownership, the project stalled because of the recession, loan defaults and a conviction on extortion charges for one of the project’s principals. But after buying the property for $29.5 million in February of last year, Walton chief executive Bill Doherty said he is three-to-four weeks from beginning construction. “This is a very real project. We’re moving fast. This is happening,” he said.