It will soon cost you money to check a bag on JetBlue if you buy the cheapest level of tickets.

JetBlue Airways Corp. said Wednesday that it will create three ticket classes beginning in mid-2015, and only the top two include at least one free checked bag.

JetBlue executives declined to give a price for the bag fee, but they said pricing would fluctuate with demand.

The airline will also add 15 seats to its Airbus A320 planes, increasing capacity to 165 from 150, and reduce average legroom to 33 inches between rows from more than 34 inches. The retrofit of cabins will start in late 2016 and take two years, executives said.

JetBlue expects that the new fare classes and bag fee will generate more than $200 million a year in operating income, and the extra seats will raise another $100 million a year.

JetBlue announced the changes as it met with investors in New York. JetBlue has been under pressure from investors to boost revenue.

In morning trading, shares of the New York-based airline jumped 27 cents, or 2.1 percent, to $12.99.

Other big airlines have added fees on checked bags since 2008.

JetBlue's move will leave Southwest as the only large U.S. airline that allows all passengers to check at least one bag for free. Southwest CEO Gary Kelly has said the lack of bag fees has attracted enough additional passengers to more than offset money that would be raised by fees.