When JetBlue Airways starts flying between Boston and Philadelphia in May, it will be the latest airline to take on US Airways in the troublesome market, putting pressure on the route’s sole carrier to lower fares.

JetBlue will make five trips a day starting May 23, with regular fares starting at $86 round trip, restoring competition to the route after Southwest Airlines dropped its Boston-Philadelphia service earlier this year, and US Airways fares doubled.