Mitsui Bank was established as a private company in July 1876, with capital of two million yen.
In ensuing decades, the Mitsui family took over numerous government industrial plants to form a major zaibatsu conglomerate, with the bank as one of its core businesses.
[3] Mitsui Bank reorganized as an unlimited partnership in 1893, and as a limited company (capital stock: ¥20 million) in 1909.
[5]: 3 World War II led Mitsui Bank to distance itself from the Mitsui zaibatsu beginning around 1937, as the bank's large balance of loans to munitions manufacturers made it vulnerable to failure should a recession occur after the war.
The merger was aimed at leveraging these synergies, as well as providing stronger competition against European banks, which were expected to consolidate following a deregulation in 1992.