Delta Air Lines has finally decided to retire its Douglas DC-9 planes this week following a long history for the iconic planes.

Delta's DC-9s were included as part of the merger with Northwest Airlines, which operated the planes. Northwest has decided in the 1990 that it will be more profitable for the company to upgrade its existing DC-9s instead of acquiring new planes. The upgraded planes were less fuel efficient than newer planes but were much cheaper than the new planes.

Northwest bought up DC-9s from other airlines, who were acquiring new jets and the company's fleet increased following its merger with Republic Airlines. Northwest's total of DC-9s reached 180 in 1997 before it started retiring the planes. Northwest still had 94 of the planes on December 31, 2007 ahead of its merger with the Delta Airlines.

Nat Pieper, Delta's vice president of fleet strategy said, "The DC-9 has been a workhorse in our domestic fleet while providing a reliable customer experience. The aircraft's retirement paves the way for newer, more efficient aircraft."

Delta is picking up 88 Boeing 717s from Southwest Airlines to replace the existing fleet of older planes.