In the early 1920s, the state of South Dakota decided to build five modern highway bridges across the Missouri river to facilitate transportation. These were at Mobridge, Chamberlain, Wheeler, Pierre, and Forest City. The Mobridge location was picked because it was the only inhabited location in northern South Dakota. While the Wheeler Bridge contact was let first, the contractor on the US-12 Bridge at Mobridge finished their project early, resulting in the US-12 Bridge at Mobridge being the first of the five state project bridges to open in November 1924.

Each of these bridges were built in a cookie cutter manner, with the basic Pennsylvania style through truss segment being the main building block. These bridges had between 3 and 5 of these truss segments. These state project bridges operated successfully until the 1950s. The major dam projects meant that all five of these bridges were to be flooded out as the Oahe, Big Bend, Fort Randall, and Gavins Point Dams were closed off and started to impound water.

In the case of the US-12 Bridge at Mobridge, adding the depth of the water and the clearance for navigation resulted in a pier height of up to 160 feet tall. While the Wheeler Bridge was successfully reused, the required number of short trusses needed at Mobridge would have resulted in a large number of very expensive piers needing to be built. It was determined that reusing the bridge was not practical. Rather, a cantilevered truss would be needed to span longer distances. The resulting bridge has 5 main channel spans. There are 2 sets of large trusses covering one span and a fourth of each adjacent span. A smaller Pennsylvania style through truss was used to connect this adjacent pair of these large trusses, plus one of these smaller through trusses was used at each end of the truss structure.

The US-12 Bridge was built with the help of the US Army Corps of Engineers. It opened in 1959. The Oahe Dam was closed off in 1961, and the reservoir was full for the first time in 1962. The resulting structure is a very big metal monster, one of the largest in the world.