The 320 South Boston Building, formerly known as the National Bank of Tulsa Building, is a 22-story high-rise building built by architect George Winkler, who also designed the Mayo Hotel. It was originally constructed as a 10-story headquarters building for the Exchange National Bank in 1917 and expanded to its present dimensions in 1929.
Covered in brick with terra cotta trim, the architectural style is Beaux-Arts. The central tower is stepped at the 20th floor with a two story arcade section, which is topped by a temple fronted section. A cupola tops the section. For many years, the cupola was illuminated by floodlights whose color changed according to the latest weather forecast. Green light meant a fair weather forecast, while red lights signified an approaching storm.
In 1933, Exchange National Bank reorganized and renamed itself as the National Bank of Tulsa. Thereafter, the building was known as the National Bank of Tulsa Building, until the bank renamed itself as Bank of Oklahoma (BOK). The BOK moved to its newly constructed BOK Tower in 1977. The NBT Building reverted to its former 320 South Boston Building name and became a general office building.