The Washington Monument in Washington DC had an even longer gestation. According to the National Park Service, "... its construction took place in two major phases, 1848-56, and 1876-84--the Civil War and a lack of funds causing the intermittent hiatus. Plans for a national monument began as early as 1783 when Major Pierre Charles L'Enfant proposed to Congress that an equestrian statue of George Washington be erected. Although the Monument was authorized by Congress, no action was taken by the time Washington died in 1799.
"His death rekindled public aspiration for an appropriate memorial to him, and John Marshall proposed that a special sepulcher be erected for the General within the Capitol itself. Lack of funds postponed construction, but Marshall persevered, and in 1833 he and James Madison formed the Washington National Monument Society. By 1836 the Society advertised for competitive architectural designs. The winning architect was Robert Mills, whose design called for a neoclassical plan which provided for a nearly-flat-topped obelisk surrounded by a circular colonnade on which would stand a statue of Washington in a chariot. Inside the colonnade statues of 30 prominent Revolutionary War heroes would be displayed.
"In an elaborate Fourth of July ceremony in 1848, the cornerstone was laid. Lack of funds and the illegal election which placed the Washington Monument Society in the hands of the Know-Nothings, a political party, caused delays. After the Know Nothings returned all records to the original society in 1858, the Civil War interrupted construction. When Lt. Col. Thomas L. Casey, Mills' successor, resumed the project in 1876, he redesigned the monument to resemble an unadorned Egyptian obelisk with a pointed pyramidion. The original design was greatly altered, producing an unembellished obelisk. The Corps of Engineers of the War Department was placed in charge of the final construction, and the monument was dedicated on February 21, 1885, and opened to the public on October 9, 1888."
Interestingly, there are other Washington monuments -- the first of which was in rural Maryland, completed in 1827 and rebuilt a few times since then. See the Maryland DNR for more information on that monument.