Frontier Airlines will add three seasonal destinations — Grand Rapids, Mich.; Indianapolis; and Dallas-Fort Worth — from Philadelphia International Airport, starting in the spring, the airline announced Thursday.

Frontier's flights to Grand Rapids and Indianapolis will start April 9; the Dallas-Fort Worth flight will ramp up on May 17. Those flights will operate two or three times a week and will run through the summer season.

With these flights, Frontier will now offer 25 nonstop destinations from Philadelphia International, the second-most behind American Airlines, Frontier spokesman Richard Oliver said.

The carrier is also offering introductory one-way fares as low as $29 for the Indianapolis and Dallas-Fort Worth flights purchased on flyfrontier.com by 11:59 p.m. on Feb. 5. The fares must be purchased for select days from April 8 through June 13.

Frontier keeps base fares low, in part, by charging fees for extras such as carry-on bags, seat selection, and even soft drinks.

These fares are catching the industry's attention, according to Morgan Stanley analyst Rajeev Lalwani.

"These [low-fare airlines] are high-gross, adding lots of capacity that has the potential to decrease overall industry pricing and increase competition," Lalwani said.

Frontier hopes to introduce low-cost fares to markets it deems "overpriced" or "underserved," notably Grand Rapids. Frontier's service there will be the only direct, "low-cost" flights to and from Philadelphia, said Oliver.

Frontier flies year-round from Philadelphia to Charlotte, N.C.; Cancun, Mexico; Denver; New Orleans; Orlando; and Punta Cana, Dominican Republic; Fort Myers, Miami and Tampa, Fla.; and San Juan, Puerto Rico.

For departures, Frontier's most popular route from Philadelphia this summer will be to Orlando, which runs up to three times daily.

The latest additions will make Philadelphia the fourth-largest city for Denver-based Frontier for departures and destinations. Frontier also flies out of Trenton, where it caters to Northeast travelers going to Florida during the colder months, according to Oliver.

In Philadelphia, Frontier will be 21 percent larger by departures and serve 50 percent more destinations in 2018 compared with last year, Oliver said.