Harley-Davidson FL is a model designation used for Harley-Davidson motorcycles since 1941, when F referred to the new large capacity 74 cui (1200cc) variant of the V-2 Overhead valve engine (″Knucklehead″) that was introduced in 1936 as Model E with 61 cui (1000cc). The presence of an additional letter L indicated higher compression, offering more power but requiring higher octane leaded gasoline. In the early 1950s, the low compression version was discontinued, and in 1952 also the low capacity EL, making FL the basic model designation of Big Twin engines for decades to come.

Additional letters marked options, like FLS for sidecar use, with adjustable fork and reverse gear. As police departments were major customers of HD, FLE for escort and law enforcement was tuned down for better idling and low speeds in cities. FLF marked the traditional foot clutch and hand shift after the move to the safer hand operated clutch and foot shifting. With a higher powered FLH option since the late 1950s, many FL were FLH.

Until the 1970s, all Harley-Davidson bigtwin FL models came with a front wheel that had the same size as the rear wheel, 5.00-16, and with large fender and wide fork to match, often combined with fairing, bags and additional chrome accessories as ″full dresser″. Customers often slimmed down the factory FL to create a ″bobber″ or even ″chopper″, removing the fender, the sheet metal and big head light, even adding slim 3.00-21 front wheels from other makes, or the whole fork of the smaller Harley-Davidson Sportster XL models that were introduced in 1957.

In 1971, the Harley-Davidson FX ″Super Glide″ was introduced as a ″factory chopper″, combining the FL frame and drive train with the smaller XL front end, basically creating the third, intermediate model range FX that sold and sells well. Since then, the FL prefix indicates that the traditional 16-inch front wheel and big fenders are used on ″Dresser″ Touring series or the traditional styling ″Heritage″ variants of the Softail series, while FX is used on sportier models or the chopper-like Softails with rather thin front wheels.

The F and FL were introduced to the Harley-Davidson model line in 1941, as large capacity versions of the Model E/EL introduced in 1936. It used a 74 cu in (1,210 cc) version of the "Knucklehead" OHV engine that powered the EL in 61 cu in (1,000 cc) form. The FL shared its frame with the EL and with the U and UL, which used a 74 cubic inch flathead engine. The FL replaced the UH and ULH, which used the same frame with 80 cu in (1,300 cc) flathead engines.

Only few Knucklehead FLs were built, as due to the war, Harley had to focus on military versions, mainly the WLA based on the simple WL flathead. HD even had to develop a BMW-like boxer model, the XA. After the war, Harley was challenged by imports of British motorcycles brands. The FL continued relatively unchanged until 1948, when it and the EL were given redesigned "Panhead" engines of the same capacities as before. These engines had several improvements over the earlier "Knuckleheads", including self-adjusting hydraulic lifters and aluminum cylinder heads to reduce weight and improve cooling. The U and UL flathead twins were discontinued in 1948, leaving the OHV EL and FL models as Harley-Davidson's large-frame motorcycles.

In 1949, a year after receiving the "Panhead" engine, the FL was given a new front suspension featuring hydraulically damped telescopic forks, replacing the leading link spring suspension of the time. These forks were standard on all big twin models for 1949, including the E, EL, F, and FL. Harley-Davidson offered the spring suspension units on these models and recommended their use on sidecar combinations, because the standard hydraulic forks do not have suitable trail.

During these front suspensions' debut model year of 1949, Harley-Davidson referred to them as "hydraulic front ends". Harley-Davidson's marketing department promoted the new suspension systems by renaming the big twin models "Hydra Glide" for the 1950 model year. This was Harley-Davidson's first departure from its policy of using simple letters to identify its models. This name was changed twice in the history of the basic large-framed E and F series models, each time signaling an improvement in the bike's technology. In addition, the Glide ending was added to the names of other models based on the FL and FX formats.^{[citation needed]}