Were there other buildings where museums are now?

Most structures built near the Mall in the 1800s are no longer standing. Markets, restaurants, small businesses, railroad stations, homes, the original US Department of Agriculture office, and two Smithsonian museums stood on and bordered the Mall. By the early 1900s, a plan was made to remake the Mall with open public parkland running down its center and space for new monuments and museums along its borders. Over the next 100 years, national memorials, monuments, and new museums displaced local businesses and old scientific buildings, and the Mall was no longer a commercial hub for Washingtonians.

Were there other buildings where the museums are now?

The Washington Gas Works stood where the National Museum of the American Indian is today. Built in 1852, the Gas Works provided gas for streetlamps and indoor lighting for homes and businesses that filled this area at that time. The works were removed in the early 1900s as architects of a new design for the Mall, the McMillan Plan, recommended clearing the Mall of residential and commercial neighborhoods to make room for a more unified and open design fit for memorials and museums.

The Gas Works wasn't the only target of the McMillan Plan. Between 1873 and 1907, railroad tracks crossed the Mall along the line of 6th Street, leading into the Baltimore and Potomac Railroad station at the intersection of 6th and B NW. The station was convenient to the Center Market, which stood where the National Archvies are today. The McMillan Plan succeeded in removing the noisy, sooty railroad and the station by 1907, ending years of debate over its place on the Mall. 

During World War I in 1918, the government built temporary office buildings on the Mall. Nearly 14,000 civilians, soldiers, and sailors came to work in the Main Navy and Munitions buildings that stood where the Vietnam Veterans Memorial and Constitution Gardens are today. During World War II, the war effort consumed available land in the city and more temporary buildings stretched across the Mall from the current National Air and Space Museum over to the National Gallery of Art, and stood where the National Museum of American History was built. Some of the "temporary" buildings were used until 1970.

To make room for new museums in the 1960s and 1970s, older temporary and permanent buildings needed to come down like the old Armory Square Hospital. It opened in 1862 to treat the Civil War's wounded and sick. This large hospital complex included 12 pavilions and overflow tents that stretched onto the Mall and served over 13,000 soldiers until the war ended in 1865. The building was used for storage and then housed the United States Fisheries Commission from 1881 to 1932. In 1964, the structure was demolished and construction began on the National Air and Space Museum in the same location.

Even old museum buildings disappeared to make way for new museums on the Mall. The Army Medical Museum and Library moved to the Mall in 1887 to a new building designed by Adolf Cluss, the same architect who designed the Arts and Industries Building. This popular museum collected and displayed medical specimens meant for researchers and tourists. By 1966, Congress transferred the Museum's National Historic Landmark status to its collection, only, to free up the land for a new museum exhibiting the modern art collection of Joseph Hirshorn. In 1968, the Medical Museum closed the building was demolished, while the staff and collections relocated to the Walter Reed Army Medical facility.