Originally conceived in the 1950s by the Dallas Texas Corporation to become Dallas's first Superblock, the building's 30,000 SF footprint occupies only a third of the 2.4 acre property, almost a full city block. To the east and west of the property, recessed plazas provided tenants with access to the exterior without the intrusion of automobiles. The 1.1 Million SF building includes five sub-surface levels tied to the Dallas Pedestrian Network and a 700-space parking garage. Excavation work on the project began in 1965 and construction was completed in 1968. Following completion, One Main was immediately sold to Equitable Life Insurance Company. Subsequent owners included RREEF and RAK Group. Today, New Orleans-based KFK Group owns the building known as "One Main".
In 2015, a large portion of the building is being converted to a hotel, The Westin Dallas Downtown. Planned to open in December 2015, the hotel will have an entrance on the ground floor while the second floor of the building will be converted to the hotel's public rooms, with the lobby in the former banking hall, as well as a restaurant and a bar. The third floor will be turned into convention facilities. The top ten floors of the building will be turned into hotel rooms, leaving nineteen floors of offices. The 32nd floor is planned to house a Starwood Preferred Guest Lounge in addition to fitness facilities and an indoor pool overlooking Main Street.