While Alco had dabbled in the diesel switcher business for years, the DL series was its first attempt at a road diesel. Like their direct competitor, EMD's passenger E-units, the DLs were powered by twin 1,000 hp prime movers and rode on six-wheel A-1-A trucks, incorporating an unpowered center axle for a smoother ride. Otto Kuhler, Alco's resident stylist, provided the DLs' "streamstyled" exterior, and in fact filed a U.S. patent for the design. His previous efforts had included the legendary Alco Atlantics for the Milwaukee Road's Hiawatha
Alco outshopped its first DL locomotive in December 1939 for the Rock Island. Diesel passenger engines were still a brand-new technology, and orders at first came in slowly. The Santa Fe and other customers placed small orders to try out the new engine. Sales were just starting to ramp up, with a New Haven order for 10 DL-109s, when World War II broke out.
With the distant war in Europe already increasing U.S. rail traffic, the New Haven had ordered its engines with dual-service gearing for freight and passenger traffic, and a maximum speed of 80 mph. When the War Production Board suspended production of new passenger diesels, the New Haven successfully petitioned for an exemption for its DL-109s, and was able to buy 50 more engines during the war. Working virtually around the clock, the fleet hauled passengers by day and freight by night for the duration of the conflict. By July 1944, the 30 engines that had been delivered so far had run up more than 5.6 million miles of service. As one New Haven superintendent put it, "They pulled the railroad through the War."