Early design work began on the narrow-body 737 in 1964. Boeing was looking to design a 50-60 seat aircraft designed for trips between 50-1,000 miles. It would be about half the size of Boeing's smallest jet, the 727, which flew up to 1,700 miles.
The plane would be comparable to the Douglas DC-9 (below) and the British Airways Corporation BAC-111.
While the initial design featured a T-tail like the 727 with engines mounted on the fuselage, the lead engineer put them on the wings instead, which meant the body could be widened to seat six people across. Thus, the 737-100 was born.
That's something that's stayed consistent across every version of the 737 through today.
The 737 launched with German airline Lufthansa, which ordered 21 of the planes in 1965.
The first one was delivered toward the end of 1967.
In 1965, a few months after Lufthansa's order, US airline United ordered 40 of the jets. However, it wanted a slightly longer version, so Boeing stretched the body a little over six feet and named the longer version the 737-200. A 737-200 that Lufthansa ordered is pictured below.
While the early 737s were assembled and tested at Boeing Field — now the King County International Airport — it was moved to the company's factory in Renton, Washington, in late 1970, where it remains today.
In 1979, Boeing began development on the 737's first major revamp, seeking to increase both the range and the capacity of the jet. The 737-300 was announced at 1981's Farnborough Airshow, and first flew in 1984. It was almost 10 feet longer than the -200, and could carry up to 149 passengers.
To power the new jet, Boeing decided to switch from the original Pratt & Whitney engines to the more-powerful CFM56-3B-1 high-bypass turbofan. There was just one problem: the 737's low ground clearance and the engine's larger diameter than on the original -100 and -200 meant that the size of the fan needed to be slightly reduced, the engine had to be moved further forward on the wing, and engine accessories had to be moved to the side to accommodate the 737-300's now-iconic non-circular air intake.
Boeing announced the even-longer 737-400 in 1986. The plane, which was stretched another 10 feet, could carry up to 188 passengers. It first flew in 1988, and entered service later that year.
The 737-500 was designed as a replacement for the -200 — it carried fewer passengers, but it incorporated the improvements of the -300 and -400 to have a much longer range. It carried 140 passengers, and entered service in 1990.
While the 737-100 and -200 remained the original models, the -300, -400, and -500 would eventually come to be known as the 737 Classic series.
In 1991, Boeing began working on another modernized update to the plane. The next-generation, or "737NG" series, was prompted by European plane-maker Airbus' introduction of its A320 narrow-body family, which rivaled the 737's dominance of the market.
Although the performance of the 737NG meant it was essentially a whole new aircraft family compared to the Classic, it kept enough important commonality with the Classic that upgrading or operating mixed fleets would be easier and more cost-effective for customers. The airframe received upgrades, the wing was redesign, and the flight deck and cabin were improved.
The 737-700 was the first to launch, and first flew in February 1997. The plane could carry up to 149 passengers, and had a longer range than previous models.
The 189-seat 737-800 came next, first flying in July 1997.
The smallest of the variants, the 132-seat 737-600, had its first flight in January 1998.
The longest version, the 189-seat 737-900, first flew in 2000. An updated version, the 737-900ER (for "extended range") could carry as many as 220 people, and first entered service in 2007.
Over the years, the 737 has been used for a variety of things, whether that was launching a new low-cost airline ...
... Serving as a military transport ...
... Or even a tactical submarine hunter like the P8 Poseidon.
It's even been used as a freighter ...
And a private business jet.
As of 2019, orders of the 737NG were still being delivered to customers.
But in the 2010s, Boeing tried to replicate the success of the 737NG with the 737 Max. But this time, it wasn't simply competing with Airbus; it was playing catch-up.
Boeing began to discuss a successor for the 737 as early as 2006, looking at both putting new, more efficient engines on an existing 737 airframe, or starting from scratch with a brand new airframe. Boeing knew that Airbus was similarly exploring an A320 replacement, but both companies were still in early stages.
It was still trying to decide in 2010, when Airbus launched the A320neo family (neo = new engine option). The jets used the original A319, A320, and A321 airframes, but used new engines that offered a 15-20% increase in fuel efficiency, consequently lowering operating costs and giving the planes longer ranges. Airbus has since released two longer-range variants of the neo family — the A321LR and A321XLR.
In July, American Airlines announced an order for 130 A320ceo and 130 A320neo jets, with an option for 365 more. It also said that it would order 100 of Boeing's not-quite-ready next-generation 737. Until that point, American Airlines had exclusively purchased from Boeing for more than a decade. Boeing knew the order was coming, and tried to quickly prepare a re-engined 737 to offer as an A320neo alternative.
In August 2011, Boeing announced the 737 Max family, consisting of four differently sized models: the 737 Max 7, Max 8, Max 9, and Max 10.
The 737 Max kept commonality with the 737NG, but used new CFM International Leap-X engines, offering improved fuel efficiency. The new engines, though, were further forward and higher up on the wings, which meant the plane could handle differently.
The first Max flight took place in January 2016, taking off from Boeing's facility at Renton. The plane was certified by the FAA in March, 2017. The first plane was delivered in May 2017. Within a year, 130 of the Max planes had been delivered, logging more than 118,000 flight hours.
Specifically, it could cause the nose of the plane to pitch upward in some situations, like low-speed flight, or flight with a high angle-of-attack, when the plane is being flown manually. To compensate for that, Boeing designed an automated software called Maneuvering Control Augmentation System (MCAS), which would automatically activate to stabilize the pitch and nudge the aircraft's nose back down "so that it feels and flies like other 737s."
MCAS would only activate when the plane was being hand-flown — meaning autopilot was off — and it was flying relatively slowly, and the nose was pointing high enough — or the angle-of-attack was high enough — to be at risk of causing the plane to stall. Crucially, though, MCAS was designed to take effect when just a single sensor showed that the angle-of-attack was high. That meant that if one of the two sensors was damaged or faulty, it could activate.
In October 2018, Lion Air Flight 610, a domestic flight from Jakarta, Indonesia, to Pangkal Pinang, crashed 12 minutes after take-off, killing all 189 passengers and crew. Initial findings suggested that MCAS activated improperly, due to a faulty angle-of-attack sensor. Boeing began working on a software fix, and issued several advisories, although the plane continued to fly for airlines around the world.
Then on March 10, 2019, Ethiopian Airlines Flight 302 from Addis Ababa to Nairobi, Kenya, crashed six minutes after taking off. All 157 people were killed. An improper MCAS activation, combined with the fact that the pilots were unable to execute the recommended recovery procedures, were blamed.
Following the second crash, the 737 Max was grounded worldwide. Multiple investigations into the plane's design and its certification have been initiated, and further potential safety risks have been found.
Despite several delays, Boeing has maintained that it plans to submit its fix in September, and expects the plane to be back in the air by the fourth quarter, although some think that early 2020 is more probable. While the details are publicly unknown, the software update is expected to make MCAS use two AoA sensors instead of one, and make it easier for pilots to override the system and adjust the trim stabilizers manually should that ever be necessary.
The coming months will show how Boeing responds to the accident, and how the market treats the plane once the grounding is lifted — whether orders resume.
