The agency managing a 23-mile Metrorail extension to Dulles Airport plans to start a $650 million bond sale this month, hoping to capture a mammoth federal subsidy offered through the stimulus package.

Under the stimulus’s “Build America Bonds” initiative — which is set to expire at the end of the year — the federal government helps municipalities pay for infrastructure projects by footing the bill for 35 percent of the interest on the projects’ debt. President Obama has proposed to make the subsidy permanent, though smaller.