Despite its size, technologically speaking, the Walnut Street bridge was not innovative for its time. Rather, it is representative of the bascule bridge technology in widespread use by the 1910s. The Walnut Street bridge does, however, make use of such mid-20th century refinements as automated controls and signals, enclosed reduction gears, and electrically operated center lock bars. The movable leaves are operated by a pinion gear engaging a segmental rack mounted to the bottom of the girders. Shafting and enclosed reduction gear sets transmit power from electric motors located on the first level of the operatorís houses. Each leaf consist of two haunched built-up steel thru girders with built-up floorbeams, steel I-beam stringers, and an open-grid steel deck. The concrete counterweights are fixed in steel frames at the heel ends of the movable spans. The leaves are locked in the closed position by automated center locks located on the exterior of the main bascule girders. Moderne-style, flat roofed reinforced concrete operaterís houses finished with scoring in a checkerboard pattern are located at opposite ends of the movable span. They have horizontally pivoted steel sash windows. The three-story house at the northwest corner has a door at roadway deck level providing access to the lower level and machinery rooms. The four-story operatorís house at the southeast corner contains the operatorís console and electrical control equipment. The console is located on the top level, which has ribbon windows on three sides providing maximum visibility for the bridge operator.
Planning for the Walnut Street Bridge began in 1952 when the Delaware State Highway Department recognized the need for improved highway access between Wilmington and points south including the newly opened Delaware Memorial Bridge near New Castle. In order to alleviate growing traffic problems and congestion on the South Market Street bridge (State Bridge NC-688), the department advocated for a new river crossing on a 3/4-mile long southern extension of Walnut Street. The project was to be funded in part by federal aid designated for use on urban highways. The department retained consulting engineers, Parsons, Brinckerhoff, Hall and MacDonald of New York to study alternative designs. The firm, chosen because of their nationally recognized expertise with urban expressways and large bridges, initially proposed a limited access expressway with high-level fixed span bridge over the Christina River, thus eliminating delays from bridge openings. That plan was passed over by the department because of the state legislatureís unwillingness to support limited-access highway legislation and the high cost of land-acquisition associated with extending the expressway over the tracks of the nearby Pennsylvania Railroad and into downtown Wilmington. The more economical option, with a movable bridge over the river and an underpass beneath the railroad, was chosen.
Construction began in June, 1954, and it was completed in May, 1957. Richard S. M. Lee, was the designing engineer, and J. C. Whiteman, Jr., was resident engineer. The primary contractor was A. S. Wikstrom, Inc. of Skaneateles, New York with the American Bridge Company of Ambridge, Pennsylvania providing the structural steel and the Earle Gear and Machine Company of Philadelphia providing the bascule machinery and gears."