The B-24's spacious, slab-sided fuselage (which earned the aircraft the nickname "Flying Boxcar") was built around two central bomb bays that could accommodate up to of ordnance in each compartment (but rarely did, as this decreased range and altitude). The forward and aft bomb bay compartments were further split longitudinally with a centerline ventral catwalk just nine inches (23 cm) wide, which also functioned as the fuselage's structural keel beam. An unusual four-panel set of all-metal, tambour-panel "roller-type" bomb bay doors, which operated very much like the movable enclosure of a rolltop desk, retracted into the fuselage, creating a minimum of aerodynamic drag to keep speed high over the target area, and also allowed the bomb bays to be opened while on the ground; the low ground clearance prevented the use of normal bomb bay doors. The occasional need for crewmen to move around inside from fore to aft within the B-24's fuselage during a mission over the narrow catwalk was a drawback shared with other designs.