England, after losing the toss, were 195 for four at tea on the first day with Bell having fallen early in the second session for 56.
But Joe Root and debutant Mooen Ali, both 43 not out, repelled Sri Lanka with an unbroken fifth-wicket stand so far worth 75.
England had struggled to 98 for three at lunch after debutant opener Sam Robson, captain Alastair Cook and Gary Ballance had all fallen cheaply in the morning session.
But Bell, in his 99th Test, was unbeaten on 41 and looking in excellent touch. Shortly after lunch, he drove left-arm spinner Rangana Herath for six to complete a 69-ball fifty also including seven fours.
Ali, one of three debutants in the England side, began cautiously but the left-hander found his form in slog-sweeping Herath for six.
Earlier, Sri Lanka captain Angelo Mathews won the toss and elected to field in the first of this two-match series against an England side playing their first Test since suffering a 281-run defeat by Australia in Sydney that condemned them to a 5-0 Ashes loss in January.
Mathews's decision yielded a quick reward as England lost both their openers inside the first half hour.
Australia-born Robson, playing on his Middlesex home ground, fell for one when he was drawn forward by a full-length Nuwan Pradeep delivery that took the outside edge and wicketkeeper Prasanna Jayawardene, diving to his right, held a fine catch.
