The 737 has been a great success story for Boeing. In fact, it has been the best selling commercial jet to date. In production for over 50 years, it has been continuously modified and updated through many variants – part of its success story. This article takes a look at the main differences through each 737 family – the Original, Classic, Next Generation, and MAX.
The Boeing 737 MAX series is the latest to join the long-running family. Photo: BoeingA long and successful history
The Boeing 737 was introduced in 1967 and has remained flying, and in production, since. To date, 10,575 have been delivered (according to data from Boeing as of February 2020), and 15,115 have been ordered. This makes it the most sold aircraft to date, but the Airbus A320 has now caught up in orders.
Part of this success has been its well-engineered design and constant evolution to meet airline demands. Back in the 1960s, the first 737 was designed to beat the competition at the time (mainly the Douglas DC-9, but also the BAC One Eleven and the Caravelle from Sud Aviation). As the 737 has moved through its variants, these improvements have continued and kept it as the top narrowbody choice for many airlines.
So, what have been the main differences for each series?
Boeing saw great success with the 707, and the 737 took much from this. Photo: airandspace.si.edu via WikimediaThe 737 Original – the 737-100 and 200
One of the main features of the first 737s was, in fact, two engines. This was a driving motivation in the development of the 737. The preceding Boeing aircraft, the 707 and 727, had both been very successful. But market attention had shifted to a more economical two engine possibility.
Boeing got ahead of the competition by placing the two engines of the 737 under the wing, as opposed to mounted at the rear of the fuselage. Critically, this allowed for a wider fuselage, giving the 737 extra passenger capacity (six across as opposed to five), and allowed for standard freight containers to be loaded. The simple conversion to freight use was popular with many airlines.
The Boeing 737-100 first flew entered service in 1967. Photo: BoeingOnly 60 737-100 aircraft were sold, and it was soon improved with the 737-200. This offered an extended fuselage, requested initially by United Airlines, and proved very popular (with 1095 sales). There was also an Advanced version, with improved aerodynamics, more powerful engines, and higher fuel capacity and range.
Other modifications were made to suit airline demand. These included a Combi version for easy conversion to freight use, and the provision of an ‘Unpaved Strip Kit’ to allow landing on gravel runways.
Boeing’s Unpaved Strip Kit was made available for the 737-100/200 aircraft only. Photo: Biggerben via WikimediaThe 737 Classic  – the 737-300, 400 and 500
Production of the ‘Classic’ series started with the 737-300 in 1984. The aim was a transformation focussed on increased capacity and better fuel efficiency. The family would be modified but keep commonality (in both design and flight operation) with earlier models.
The main difference was adding new engines. The 737-300 uses CFM56 turbofan engines. This increased thrust (up to 23,500 lbf compared to 16,400 lbf for the Pratt & Whitney engines on the 737-200), but led to a few design challenges. The larger engine diameter, and low ground clearance of the 737, meant they were placed ahead of the wing.
The 737-300 was the highest-selling of the original and classic series. Photo: Dylan Ashe via WikimediaThe 737-300 offered a small increase in capacity (up to a maximum of 149 compared to 136 for the 737-200), from an extension of the fuselage around the wing. Other structural improvements included:

An increase in wingspan and extension of the wingtips, offering improved aerodynamics
Resdesign of the tailfin
Several cabin improvements (based on features developed for the Boeing 757)

The 737-400 continued this expansion. Stretching the 737-300 around three meters increased the capacity to 188. In contrast, the 737-500 was a smaller model. This went back to the size and capacity of the 737-200 but offered the improvements in design and efficiency of the Classic series.
The last member of the Classic series, the 737-500, offered a direct alternative for aging 737-200 aircraft. Photo: BriYYZ via Wikimedia CommonsThe 737 Next Generation – the 737-600, 700, 800 and 900
The Next Generation (NG) series was launched in 1993 (first flying in 1997). This came about mainly to address growing competition from the Airbus A320 family, and high fuel prices at the time.
The series offered several improvements over the Classic aircraft, including better fuel efficiency, extended range, and larger capacity variants (up to 215 passengers). Changes to the aircraft included:

Upgraded CFM56-7 series engines, with improved fuel efficiency
A redesigned wing, with increased span and area (and allowing increased fuel capacity)
Improved digital cockpit
Interior cabin improvements, including larger bin space (with several updates based on 777 designs)

The revised cabin interior on the 737NG. Photo: BoeingThe 737-600 is the smallest of the family, with a capacity of 149 passengers. The 737-700 is stretched by around 2.4 meters, and also offered a convertible cargo option, the 737-700C.
The 737-800 is stretched further than the 737-700, making it a good replacement for the 737-400. It takes passenger capacity up to 189. This is increased further on the longest model – the 737-900ER, with capacity up to 220 (the 737-900 is limited to 189 due to only having one less set of exit doors).
The 737-800 has become Boeing’s best selling 737 model. Simple Flying looked in detail at this, discussing how compromise has made it such a success. It offers an excellent combination of range and capacity, not the largest of either, but a very versatile option for many airlines.
The 737-800 has been very successful as a great all-round performer. Photo: Getty ImagesThe 737 MAX Series
The fourth generation of the 737 family was launched in 2011. The 737 MAX series competes with the A320neo family (launched in 2010), and once again focusses on improving efficiency.
It uses new, and more efficient, CFM International LEAP engines, and includes several aerodynamic modifications, including distinctive winglets.
Like the NG series, there are four MAX variants of different sizes. These generally offer increased passenger capacity and range over the NG variants. Simple Flying took a closer look at these differences in a previous article.
The 737 MAX with LEAP engines and winglets. Photo: Getty Images.The 737 MAX, of course, has suffered from its grounding in 2019 following two crashes. It has still proved popular, though, with 4,741 orders (based on Wikipedia order data).
This shows its popularity and appeal to airlines well. Since just 2011, it has already almost matched 737-800 orders (and this has been on sale since 1993). If Boeing can overcome the problems, the 737 MAX could be a great success.
The next 737?
While the problems with the MAX series and the aviation slowdown in 2020 could well change plans for new aircraft, Boeing already has plans for a new aircraft.  This was initially planned by 2030.
It would likely be a composite airframe construction, offering a lightweight fuel-efficient construction. Potential modifications also include an elli[itical cross-section, offering a wider cabin with the possibility of two aisles. Widening the cabin was how the 737 Original got its lead, so it will be interesting to follow whether this happens again.

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