Delta Air Lines said it is expanding its premium economy class to many of its domestic flights, adding the section with extra legroom to its entire mainline fleet and larger regional jets.

Atlanta-based Delta earlier this year added the section with extra legroom to long-haul international flights, and it said the move was successful based on customer bookings.

The airline now plans to add the seats by summer to all its more than 550 mainline aircraft and all 250 regional jets that have first class. That covers many of the airline's domestic flights as well as all its international flights.

"Our goal is to make sure premium customers are sitting in premium seats," Delta chief executive Richard Anderson said in a written statement.

Delta said it will remove a "negligible number" of seats from planes to make room for the three to five rows of the "economy comfort" section -- which has the same physical seats but more space between rows. The introductory price to sit in the premium economy section will be $19 to $99 each way on top of the fare. The new section will have about three inches of extra legroom and will come with priority boarding.

Not everyone will have to pay extra. The top three tiers of elite frequent-flier members on Delta -- diamond, platinum and gold -- can all reserve the economy comfort seats for free when they book their flights, as well as those who book the highest priced economy class tickets. Silver elite frequent fliers will get a discount for the seats or can get them for free when they check in if the seats are available.

Unlike on long-haul international flights, the economy comfort section on domestic flights and shorter international flights will not get free alcoholic beverages nor will the seats have additional recline. The lack of additional recline is because passengers on the shorter flights don't have as much of a need to sleep and have more of a need to use laptops comfortably, Delta spokeswoman Chris Singley said.

The first of the new round of converted aircraft will be a Boeing 757, which will have the premium economy section starting in November. Delta will not begin charging for the seats until the conversions are completed. Seats will be available for bookings starting in late spring for travel in summer.