The B-17 was the Air Force's first aircraft made predominantly of metal. Its only non-metal parts were its control surfaces—the vertical and horizontal stabilizers on the rudder and the ailerons on the wings (the movable part used to control roll). To keep them lightweight, these control surfaces were still made of linen- and cotton-covered wood, which was coated with a highly flammable varnish called dope. Used to tauten, stiffen, and waterproof the fabric covering, dope was one of the primary materials stored in this one-story, glazed brick-block building. Set off by itself to protect the rest of the base in case of an explosion, this fire-resistant storage building was the base's only masonry structure. Inside, two rooms separated by a thick masonry wall isolated the coal-burning stove used for heat from the main storage room. Planes came fully assembled, but ground crews responsible for maintaining the aircraft used dope to repair planes damaged during training. These ground crews took what they learned about maintaining the training planes with them when they deployed to Europe and North Africa, where they repaired aircraft damaged in combat.