The Catalina is a twin-engined high-winged monoplane flying boat with retractable wing tip floats. It features an almost cantilevered wing mounted above a shallow but broad hull on a central pylon housing the flight engineer. The wing has a rectangular centre section and tapered outer panels, all of stressed-skin all-metal construction, though the ailerons and trailing edges are fabric-skinned. A unique feature is the wing-tip floats, which are mounted on pivoted frames that can be retracted electrically so that in flight the floats form the wingtips. The hull is also all-metal, with a broad semicircular upper surface.
The bow has a mooring compartment and transparent sighting window, with a roller blind giving seawater protection. A turret all-round window is fitted in the upper bow. The two pilots sit side-by-side in a wide cockpit with large windows all round. Left and right gunner stations have blister windows on the waist of the hull behind the wing. The tail is of a tall design, with the horizontal tail mounted well up the single fin. The power plant comprises a pair of two-row Pratt & Whitney Twin Wasp engines neatly cowled on the centre section with cooling gills. The engines drive Hamilton variable-pitch propellers.