The Knox Building's facade remains largely as it was originally designed, with decorated limestone cladding, a cornice above the sixth floor, and a mansard roof.

The bank acquired the neighboring lots in the 1970s and hired Attia & Perkins to design a tower to house its new world headquarters, which would wrap around the Knox Building.

[13] The modern skyscraper includes the former Kress Building at the corner of Fifth Avenue and 39th Street, which was originally designed in the Art Deco style and built in the mid-1930s.

[14][15][a] An annex on 20 West 40th Street replaced the Willkie Memorial Building, designed by Henry Janeway Hardenbergh and built in 1905.

[14] The ten-story Knox Building,[16][b] at the southwest corner of 40th Street and Fifth Avenue, was designed by John H. Duncan in the Beaux-Arts style.

The inner bays on 40th Street contain three wide openings on the tenth floor, each designed with nine narrow vertical glass panes.

According to Attia & Perkins, this allowed the HSBC Tower to rise as a distinct entity from the mansard roof of the Knox Building.

The facade of the former Kress Building was replaced with a granite cladding containing rose, beige, and gray hues.

The serving counters of the dining room were adapted from the green marble countertops of the Knox Building's original banking hall.

[38] In 1901, Edward Knox acquired the Kip mansion at the southwest corner of Fifth Avenue and 40th Street,[39] across from the vacant site of the Croton Reservoir, later the New York Public Library Main Branch.

[39] Knox received title to the site that April for $450,000,[43][44] and he hired John Duncan to extend and remodel the old Kip mansion for commercial use.

[37] The store was described as having carved and paneled walnut decoration, plate-glass mirrors with brought-iron frames, bronze-and-marble counter tops, large chandeliers, and gold-plated cabinets.

[67] Other lessees in the 1930s also included Brown & Wells Inc.,[68][69] hairdressers Julian Inc.,[70] and publishers Parfumerie St. Denis Inc. and House of Peters Inc.[71] In 1964, the Knox heirs sold the building to a group headed by Martin Ackerman.

With that acquisition, the Ackerman group filed to incorporate the Republic National Bank of New York within the Knox Building.

[77] The New York City Planning Commission approved an eleven-story annex of the Republic National Bank Building on Fifth Avenue, south of the original structure, in 1974.

Republic was obligated to design the annex to "harmonize" with that of the New York Public Library Main Branch, an official city landmark.

[78] By the late 1970s, the bank planned to erect a world headquarters tower on the site to consolidate operations at 15 buildings across New York City.

A mirror-image tower on the northeast corner of Fifth Avenue and 39th Street, which Republic had proposed on the site of a Woolworth store, had been disapproved the previous March.

[81] The project was to be the largest development on Fifth Avenue south of 42nd Street since the Empire State Building, which had been completed five decades earlier.

The 12th story and higher were leased to tenants such as the Federal Deposit Insurance Corporation, Aetna, Krungthai Bank, and Hewitt Associates.

[86] Fox & Fowle proposed a 12-story building at 20 West 40th Street, adjacent to the Republic National Bank Tower, in 1989 or 1990.

In exchange, the bank proposed restoring the Knox Building's Fifth Avenue entrance marquee and seventh-story railing.

[22] After multiple revisions, including a stipulation that the facade's material be changed to brick,[6] the LPC approved the project in November 1993.

[88] The City Planning Commission also gave Republic a waiver to permit the construction of four additional stories, covering 60,000 square feet (5,600 m2), in exchange for maintaining the Knox Building.

[89][90] Giuliani's administration also offered to permit foundation work for the annex to proceed before the requisite Uniform Land Use Review Procedure for the site was completed.

This work had to be done with minimal interruption to electrical service, since the building was occupied for the full duration of the renovation, and all non-emergency street excavations in Midtown were halted each winter.

[96] Tops Appliance City, an electronics store, leased space on the ground and second floors of the National Republic Bank Tower in 1998.

[134] PBC's parent company Discount Investment Corporation announced in March 2024 that it wanted to pay off $400 million in debt on the tower by selling off bonds.

[138] When the Republic National Bank Tower was being constructed, Paul Goldberger praised the Knox Building as a "lyrical classical gem".

[139] When the second annex was built in the 1990s, Peter Slatin wrote for The New York Times that the first tower "exemplifies the now largely discarded approach to designing buildings that clearly denote their separateness from their historic setting".