Allegiant Air today announced it has added three new nonstop destinations from Des Moines — Austin, Texas; Chicago; and Memphis, Tenn. — as part of a 44-city expansion that the airline said is the largest network expansion in its history. The announcement comes just 60 days after Allegiant designated Des Moines International Airport as its 20th base of operations with Des Moines-based aircraft and crews. The three new twice-weekly flights begin booking today for travel beginning May 21. “These three cities are going to open up incredible vacation destinations,” said Hilarie Grey, Allegiant’s director of corporate communications. Des Moines is among just six airports for which Allegiant has added its new service to Chicago’s Midway Airport. Allegiant in November added Nashville, Tenn., as a direct flight, beginning Feb. 13. With this newest service expansion at DSM International, Allegiant now has 12 nonstop destinations from Des Moines, the most of any airline serving Des Moines. In total, DSM International now has nonstop flights to 25 cities, a 55% increase from 2011, airport Manager Kevin Foley said.