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, of Bell & Kent of Council Bluffs, Iowa. In order to have their design built, Bell & Kent relocated their office to Helena. 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 & 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 was extended by the addition of two new wings on the eastern and western sides. This work was executed by Link & Haire, architects of Butte, with F. M. Andrews & Company of New York as consulting architects.
The most notable feature inside the center of the Capitol building is the massive rotunda, with four circular paintings surrounding it. These paintings, painted for the Capitol opening in 1902 by the firm of F. Pedretti's Sons, depict four important archetypes of people of Montana's early history: a Native American, an explorer and fur trapper, a gold miner, and a cowboy. The Pendretti brothers provided additional commissioned artwork in the Senate and Old Supreme Court Chambers. The southern arch of the rotunda features the semi-elliptical painting Driving The Golden Spike, painted by Amédée Joullin. The former State Law Library, now a set of committee rooms, features ten Montana landscapes created by Ralph E. DeCamp. The House of Representatives Lobby features six scenes depicting significant events in early Montana History by Edgar S. Paxson. The most significant piece of art in the Capitol is by Montana's famous Western artist Charles M. Russell. The 1912 painting, titled Lewis and Clark Meeting the Flathead Indians at Ross' Hole, is long and twelve feet high. It depicts the explorers Lewis and Clark meeting Montana's Bitterroot Salish people upon their return across the Bitterroot Mountains from the Pacific Ocean. It is now displayed above the Speaker's chair in the House of Representatives' chamber.
