Boeing delivered the 5,000th 737 to Southwest Airlines on February 13, 2006. The 6,000th 737 was delivered to Norwegian Air Shuttle in April 2009. The Airbus A320 family has sold more planes than the 737NG over the past ten years. However, this total includes sales for the A321 and A318. The A321 has been the rival of Boeing's 757 and the A318 has been the rival of the 717. The Next Generation series helped make sure the 737 stayed the bestselling airliner family since it first flew.