The Boeing 737-800 is a stretched version of the 737-700. It replaced the 737-400. The Boeing 737-800 competes primarily with the Airbus A320. The 737-800 seats 162 passengers in a two-class layout or 189 passengers in a one-class layout. The 737-800 was launched on September 5, 1994.[4] Launch customer Hapag-Lloyd Flug (now TUIfly) received the first one in April 1998.[53]Following Boeing's merger with McDonnell Douglas, the 737-800 also filled the gap left by Boeing's decision to discontinue the McDonnell Douglas MD-80 and MD-90 aircraft. For many airlines in the U.S., the 737-800 replaced aging Boeing 727-200 trijets.The 737-800 burns 850 US gallons (3,200 L) of jet fuel per hour—about 80 percent of the fuel used by an MD-80 on a comparable flight, while carrying more passengers.[54] The Airline Monitor, an industry publication, quotes a 737-800 fuel burn of 4.88 US gal (18.5 L) per seat per hour, compared to 5.13 US gal (19.4 L) for the A320.[55] In 2011, United Airlines— flying a Boeing 737-800 from Houston to Chicago—operated the first U.S. commercial flight powered by a blend of algae-derived biofuel and traditional jet fuel to reduce its carbon footprint.[56]In early 2017, a new 737-800 was valued at $48.3 million, falling to below $47 million by mid-2018.[57][unreliable source?] By 2025, a 17-year-old 737-800W will be worth $9.5 million and leased for $140,000 per month.[58][unreliable source?]As of May 2019, Boeing had delivered 4,979 737-800s, 116 737-800As, and 21 737-800 BBJ2s, and has twelve 737-800 unfilled orders.[5] The 737-800 is the most common variant of the 737NG and is the most widely used narrowbody aircraft.[59] Ryanair, an Irish low-cost airline, is among the largest operators of the Boeing 737-800, with a fleet of over 400 737-800 aircraft serving routes across Europe, Middle East and North Africa.[60]