When New York’s J.P. Morgan Chase announced in January 2004 that it was buying Bank One, formerly the First National Bank of Chicago, there was much civic hand-wringing over the loss of the city’s last hometown banking giant. This week, employees at Chase Tower celebrated the 20th anniversary of the deal’s closing with a commemorative brochure and free donuts. While the $58 billion Bank One acquisition moved the headquarters to New York, it also saved potentially thousands of jobs and put Chicago at the center of what would become the nation’s largest bank. “We ended up being a great bank,” said Jamie Dimon, 68, chairman and CEO of JPMorgan Chase, and the architect of the merger.