Up swung the doors of ‘the vast factory hangar at the Glenn L. Martin plant the other day, and two tractors towed into daylight the world’s largest flying boat. Almost ready for its first dip, this battleship of the air - destined for the U. S. Navy - weighs 70 tons. Similar in some specifications to the giant land plane recently built for the army by Douglas, the Martin XPB2M-1 has a wingspread of 200 feet and is powered, like the Douglas, by four 2,000 horsepower Wright engines.