First National, a subsidiary of Pennsylvania-based F.N.B. Corp., announced plans last September to open a regional headquarters and branch employing 50 people at 300 E. Lombard St. The rapidly expanding bank, which recently unveiled its logo atop a previously signless tower, is officially opening the headquarters Monday. Vincent J. Delie, F.N.B.'s president and CEO, has said when the company expands into a major metro market, it looks to be in the central business district surrounded by businesses and potential customers.