Boeing 737-400

The 737-400 was the second version of the 737 Classic series and the longest of the three. Boeing conceived it in 1985 to fill the gap in its line-up between the 737-300 and the 757-200 and to compete with the Airbus A320 and the McDonnell Douglas MD-80. In June 1986 it announced development with a launch order from Piedmont Airlines for 25 aircraft with options on 30 more.

The -400 stretched the 737-300 fuselage a further 10 ft (3.05 m) to carry up to 188 passengers. The stretch required two changes of its own: a tail bumper to prevent tailstrikes during take-off, an early issue on the 757, and a strengthened wing spar. From the rest of the Classic series it inherited the big changes over the original 737. The CFM56-3 engines, far more economical and quieter than the old turbofans, did not fit under a wing as close to the ground as the 737's, a legacy of its 707-derived fuselage; Boeing and CFM International placed them ahead of the wing and moved the accessories to the sides of the pod, giving the intake its distinctive flat-bottomed shape, nicknamed the "hamster pouch". The wingtip was extended by 9 inches (23 cm) and the slats and flaps were adjusted; the flight deck could have optional electronic flight instruments, the cabin took improvements from the 757 and the tail gained a dorsal fin. The series also introduced the Speed Trim System, which adjusts the stabilizer automatically at low speed, low weight, aft centre of gravity and high thrust with the autopilot disengaged, the forerunner of the 737NG's flight envelope protections and of the 737 MAX's MCAS.

The prototype rolled out on January 26, 1988 and first flew on February 19, 1988. On September 15 that year it entered service with Piedmont Airlines. Throughout the 1980s the Classic series attracted large orders in the United States and Europe, exceeding those of earlier 737 models; the -400 reached 486 deliveries, behind the 1,113 of the -300 and ahead of the 389 of the -500. Its main operators were US carriers, small national airlines and charter carriers.

In the 1990s, United Airlines' purchase of the A320 prompted Boeing to turn the slower, shorter-range -400 into the rewinged, more efficient and longer 737-800. For a while both generations shared the production line, until on February 28, 2000 the last two 737-400s, which were also the last Classics built, were delivered to CSA Czech Airlines. In the Next Generation family it was replaced by the 737-800.

The -400 had a second life as a freighter. The 737-400SF (Special Freighter) or 737-400BDSF (Bedek Special Freighter) is a -400 converted to haul cargo; Alaska Airlines was the first to convert one of its own, able to carry 10 pallets, and turned five more into fixed combi aircraft, half passengers and half freight, which it retired in 2017 and replaced with 737-700Fs. In November 2018 VX Capital launched an asset-backed security on 35 737-400 freighters valued at $250 million, to be leased for $100,000 per month each.

In July 2019, 261 737-400s were still in commercial service out of a total of 692 Classics. As of January 2021 the Classic series as a whole had suffered 64 hull losses with 1,298 fatalities; a Boeing analysis of the period 1959–2017 gave it a hull loss rate of 0.71 per million departures, against 0.17 for the Next Generation and 1.75 for the original 737.