The Union National Bank building at Fourth Avenue and Wood Streets was constructed in 1906. Designed by the architectural firm of MacClure & Spahr, the building displays an austere version of the Classical Revival style, executed in gray granite. MacClure & Spahr was one of the most prominent architectural firms to practice in Pittsburgh during the first two decades of the 20 th century. The firm's other work included Downtown office buildings and homes for Pittsburgh's industrial leadership, including the Heinz family. The adjoining Commonwealth Trust Company building, built simultaneously with the Union National Bank, was later joined with its neighbor. The Commonwealth Trust building was designed by Frederick Osterling, whose work also included the nearby Arrott Building.

Fourth Avenue in Downtown Pittsburgh became the heart of the city's financial district in the mid-19 th century. The Bank of Pittsburgh became the first financial institution to locate on Fourth Avenue in 1835, when it moved to a new building in the Greek Revival style. By the beginning of the Civil War, nine of the approximately 17 banks in Pittsburgh were located on Fourth Avenue. The Dollar Bank building, now the oldest bank in Downtown Pittsburgh, was built in 1871 to the designs of Philadelphia architect Isaac Hobbs. An 1872 plat map depicts the new Dollar Bank on Fourth Avenue; its neighbors included long-forgotten institutions such as the Merchants and Manufacturers Bank, the Mechanics Bank, and the Farmers Bank. As Fourth Avenue grew as a financial center, more banks were built and at the Burke Building at 209 Fourth Avenue, built in 1836 as Pittsburgh's first office building, was refitted to house a bank in the late 19 th century. As Fourth Avenue became dominated by financial services, Grant Street evolved into a lawyers' corridor, Firstside and the Penn-Liberty area contained increasing numbers of wholesale businesses because of their nearness to rail and water transportation, and central Downtown housed a number of department and specialty stores.

Fourth Avenue took on much of its present appearance during the first decade of the 20 th century, when several of its most prominent buildings were constructed. The building boom was abetted by the development of steel frame "skyscraper" construction in the 1890s; for the first time, the height of a building was not limited by the weight that brick bearing walls could carry. The wave of corporate mergers that swept Pittsburgh's steel, glass, and beer industries at the turn of the century also contributed to the demand for Downtown office space among financial industry firms and others. Many of the merged companies, unlike their privately held predecessors, were publicly traded, leading to a significant increase in the number of stockbrokers.

Buildings constructed on Fourth Avenue during the 1900-1910 boom included the Arrott Building, in 1901-1902 and the Colonial Trust Company Building of 1902; both were designed by Frederick Osterling, who had already made his mark in the immediate neighborhood with the Times Building at 346 Fourth Avenue and the Marine National Bank at 301 Smithfield Street. Other significant banking buildings that became part of the Fourth Avenue streetscape just after the turn of the century were the Stock Exchange Building by Charles Bartberger (1903), the Pittsburgh Bank for Savings Building at Fourth Avenue and Smithfield Street (1903). All of these followed the Pittsburgh Engineers Building (originally the Union Trust Company), built in 1898 and designed by Daniel Burnham of Chicago.

During the first half of the 20 th century, banks and other financial institutions employed far fewer workers than they do today. In addition to its ornate lobby and its banking offices, the Union National Bank building contained numerous offices on upper floors that were occupied by dozens of insurance agents, stockbrokers, real estate agents, accounting firms, and lawyers, all clustered in Pittsburgh's financial district. Other offices in the building housed manufacturing and contracting firms that needed Downtown offices. The basement, like that of countless other banks, contained a barbershop. Later in the 20 th century, as financial institutions grew, merged, and increased workforces, they typically reclaimed upper floors in their buildings.

The Fourth Avenue Historic District was listed on the National Register of Historic Places in 1985, with the Union National Bank building a contributing building within the district.