A design competition for the building was conducted in 1896. The commission selected a design by George R. Mann as the winner. In 1897, after it was found that the Commission was planning to scam money from the building project, it was disbanded and a second Capitol Commission was convened. The new Commission abandoned Mann's plan as being too costly, and had a second design competition, won by Charles Emlen Bell and John Hackett Kent. While Mann's building was never built in Montana, it was selected later as the basic design for the Arkansas State Capitol.

The winning design by Bell and Kent had been altered already during the construction phase, when in 1901 the commission asked for the structure to be made more imposing by increasing the height of the dome. Kent opposed the changes, as his original low spherical dome was meant to be "pure Greek", but Bell advocated the commission's request.  Between 1909 and 1912, the building has been extended by addition of two new wings on the eastern and western sides.