For the majority of its history United had been one of the world's largest airlines, however it was not an international airline and its fleet of 747-100s hastily bought in the late 60s were primarily operating services to Hawaii or transcontinental domestic routes. Deregulation changed all this for United - able to flex its muscles domestically and supplant the traditional US flag carriers it entered the 1990s ready to expand globally and to achieve this it needed new widebody aircraft.
Indeed during 1991 United more than doubled its European network as it acquired Pan Am's London hub and in 1992 it began its South American network with flights to Caracas, Rio de Janeiro, Sao Paulo and Buenos Aires. The second batch of 747-400s began to join the fleet in August 1996 with N193UA but in the end only 20 extra frames were taken on for a combined fleet of 42 747-422s. Deliveries continued until N128UA on May 12, 2000. The completion of 747-400 deliveries roughly correlated with the retirement of the last 747-100s and 200s. The last of the 747-238Bs seem to have been withdrawn in December 2000.
By that time United was entering into troubled waters and following Sept 11 it entered bankruptcy, which involved dropping around ten of the older 747-400s. Despite gradual retirements there are still 21 747-400s in service with United in the post-Continental merged airline. At present United plans to continue flying the 747-400 until October 2018 when N177UA is withdrawn. Though now even the 747-400 is an old type it more than any other is responsible for United embracing its opportunity to become a truly global airline during the formative early 1990s.