Bello made all but two of his birdie putts from about 6 feet or closer. He was helped by a pair of 30-foot birdie putts near the end of his round and a 10-foot par save at No. 9 on his final hole.
Keegan Bradley had a 67 and Graeme McDowell a 68.
Only five players from the morning wave broke 70. Defending champion Rory McIlroy played in the afternoon.
Jason Day, the last player to win at Bay Hill before Palmer died in 2016, withdrew after six holes because of a back injury. Day was coming off a three-week break