Boeing's best-selling 737 airliner just celebrated 55 years of production. See inside the factory where the plane is built.

This year marks the 80th anniversary of Boeing's Renton factory in Washington state and 55 years of the planemaker's 737 program.

Source: Boeing

Production at the 1.1 million-square-foot factory started with tankers and bombers during World War II. The first-ever aircraft produced at Renton was the XPBB-1, and only one was built, giving it the nickname the "Lone Ranger."

Source: Boeing

Other famous military planes built at Renton include the Boeing B-29 Superfortress and the Boeing 367-80, nicknamed the "Dash 80."

The Dash 80 led to the production of the first jet-powered tanker, the KC-135 Stratotanker…

…and the Boeing 707 commercial airliner, which ushered in a new era of international travel.

About 30% of the world's commercial jetliners, including the Boeing 707, 727, 737, and 757, were built at Renton — which is about 14,500 airplanes.

Source: Boeing

In 1967, the production of Boeing's best-selling 737 began. The company built two "original" 737 models, including the -100 and -200…

Source: Boeing

…and three "classic" variants, including the -300, -400, and -500.

Source: Boeing

From 1997 to 2019, Renton built Boeing's Next Generation 737 aircraft: the -600, -700, -800, and -900.

Source: Boeing

Boeing's fourth-generation plane, the 737 MAX passenger jet, began production in 2015 at Renton. Three models — the MAX 8, MAX 9, and MAX 8200 — are currently in service with airlines around the world.

Source: Boeing

However, the MAX program proved to have problems, with two planes crashing due to a software design flaw that led to a worldwide grounding of the jet.

Source: Insider

The fallout of the tragedies, which killed 346 people, forced Boeing to reengineer its stall prevention system.

Since the Federal Aviation Administration signed off on the plane's airworthiness in November 2020, airlines have started flying them again.

Boeing is also continuing the development of the 737 MAX 7 and 10, which are awaiting certification.

Source: Boeing

Insider toured the factory to see the 737 MAX final assembly line and learn about the process — take a look.

The tour was led by Dennis Eng, Boeing's director of 737 program business operations. He explained the factory is split into three assembly lines — east, west, and center.

Currently, the west and center lines are active, and Boeing plans to activate the east side once it has enough personnel.

While Eng did not specify what the current monthly MAX output is, he said Boeing's goal is to produce 31 planes per month.

Walking through the factory, Eng said the planes go through 10 different flow days during final assembly, which includes things like hydraulic, wiring, and systems installation, joining the wings to the fuselage, installing the interior, and testing.

The fuselage comes complete from Wichita, Kansas, and is built by Spirit AeroSystems. It is then transported via rail to Renton where it is joined to the wings.

To join the body is a complicated process, according to Eng, who said workers use cranes and other tools to move parts around the massive building.

Because of the complexities, the movements are done during one of the three shifts employees work.

When installing economy class seats, Boeing uses a modified hay bail. This process is done during the first shift and completed while the fuselage is still open in the back to make it easier.

The planes had their engines and landing gear by the last flow.

After final assembly, the jets will complete flight testing out of Boeing Field, be painted in their respective livery, and delivered to their final owner.

Eng explained Boeing has reimagined its Renton factory over the years to create more efficient processes. Specifically, the planemaker has developed a system that better stages parts to ensure employees have the exact tools they need for each flow.

This is done by assembling flow-specific kits that are positioned at each station. This eliminates the need for workers to walk back and forth between the plane and the tool room during shifts.

The system improves quality and safety and decreases plane damage, according to Boeing.

We saw a few completed 737 planes at the factory, including Southwest Airlines, Alaska Airlines, and a United MAX 10, which Eng said is a test aircraft. It is rare to see the -10 in a completed livery.

During our grand tour of Boeing factories, Insider also spent time in Everett where the manufacturer is building cargo planes and other passenger jets, like the 777 and 787.

There were several seemingly complete 787 aircraft in the factory, but Boeing has been unable to deliver the jet since May 2021 due to safety concerns from the Federal Aviation Administration regarding gaps between fuselage sections.

Boeing submitted incomplete documents for its grounded $12.5 billion 787 Dreamliner jets, authorities say

As a special treat, we also saw one of the last 747 planes ever to be built, which is a 747-8F going to cargo carrier Atlas Air. The airline has three on order, which will all be delivered by the end of 2022.

Boeing's iconic 747 will leave the assembly line for the last time this year. See one of last jumbo jets the planemaker will ever build.

The completion of the final three jets will mark the end of a 54-year era that revolutionized air travel by making international flights affordable for the first time.

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