In order to handle the introduction of new vehicle models and changes to existing models, the Production Engineering Group worked to develop a flexible and innovative welding line for vehicle bodies. These efforts culminated in August 1985 when the flexible body line (FBL) began operating in earnest at the company's Tsutsumi Plant.

Until the beginning of the 1980s, only two to three vehicle models could be assembled on the same body line, and when redesigns occurred, lines had to be stopped for two to four weeks while specialized machinery was replaced. Under the FBL, however, when using robots to weld the main components, including the vehicle underbody and side panels, to the vehicle, general-purpose machinery was combined with specialized machinery to allow work on a wide range of models. Accordingly, when model redesigns were introduced, only the machinery specific to that vehicle type needed to be converted or switched. There was no need to stop the entire line, the lead time for production preparation decreased, and less plant space was required. Although in principle the new lines could handle any number of models at once, for practical reasons, the setup was made to handle four models at a time.

The development project kicked off in 1983, and following the construction of prototype lines and testing on actual lines at the Tsutsumi Plant over the next year and a half, the new lines commenced full operation in 1985. The FBL was subsequently deployed at Toyota's Japanese plants and vehicle body suppliers, and was introduced overseas for the first time at Toyota Motor Manufacturing Kentucky, Inc. (TMM; now TMMK) in the United States, which began operations in 1988. FBL went on to become a key technology as Toyota expanded its overseas plant network.

By the end of 1997, the FBL, which was awarded the Okochi Memorial Foundation Production Prize in 1990, had been introduced in 28 places in Japan and overseas. FBL technology paved the way for the next-generation global body line (GBL) technology developed in the late 1990s.