Bethesda-based commercial real estate services and finance company Walker & Dunlop Inc. finished 2017 as the largest Fannie Mae DUS lender by volume of multifamily loans originated. Walker & Dunlop moved up in the 2017 rankings from second place last year. Fannie Mae provided $66.1 billion of capital to the multifamily housing market in 2017, up 20 percent from 2016.