Three weeks after Southwest Airlines said it would start flying nonstop from Boston to Phoenix in the fall, JetBlue Airways plans to announce today that it will also be jetting into the Arizona capital from Logan International Airport.JetBlue’s daily Phoenix flight starts Sept. 2, five days before Southwest’s, and is on sale for $99 each way until Monday  good for travel through Nov. 17. The regular prices will start at $129 each way, the same as Southwest’s lowest fare between the two cities.

Phoenix is JetBlue’s 38th nonstop destination out of Logan, the most of any airline. JetBlue has been expanding steadily in Boston, increasing its number of daily departures 30 percent this summer and recently announcing new service to Washington, D.C., and Sarasota, Fla., both beginning in November.Boston’s working, said Scott Laurence, JetBlue’s vice president of route planning. If it continues to work, we’ll continue to grow.