The 1930's saw Harley-Davidson break more speed records.
In 1932 the three-wheel Servi-Car was introduced which would become a common police and commercial vehicle.

In 1936 the Knucklehead was launched and the Panhead followed in 1947. 
This machine made the Harley-Davidson American's motorcycle of choice.
During World War II the Harley took an active roll. Civilian production was just about suspended and all efforts were concentrated on war production.
Once the war was over in 1945 civilian production commenced again.
In 1949 the Hydra-Glide was introduced using hydraulic front brakes.

The 1950?s were not a good time at Harley-Davidson.
The British with their Triumph and BSA motorcycles had 40% of the market.
These were smaller, lighter and in some cases faster bikes.
This problem was met head on and in 1957 with the introduction of the Sportster.
The English stayed on the scene until the Japanese bikes hit the market in the early 60?s.
They were small, under powered but cute and did a lot to bring motorcycle to the attention of
the general public.
However they didn't stay small and cute very long.
Companies like Honda, Kawasaki and Suzuki learned real fast and made some real 
fast motorcycles.
Harley was almost put under at this time but they fought back.
In 1969 Harley Davidson merged with American Machine and Foundry Company.
This merger gave them needed cash but produced some poorly built bikes and Harley?s reputation suffered greatly.
Harley-Davidson did survive and in 1971 the ?Cruiser? was born. It was a sporty looking bike with the frame and power train of the FL series.
This motorcycle was a great success and enabled Harley-Davidson to move to a 400,000 square foot plant in York, Pennsylvania.
The 1970's saw Harley win a great many races including four consecutive years of winning the AMA Grand National Championships in dirt track racing.
I will leave the history of the Harley-Davidson at this point with saying that from here on Harley-Davidson never looked back.
There are many other good web sites that will take you beyond the 1970's

1948 was the first year for the " tin top" overhead valve version of the bike.
These were offered in both 61 CI and 74 CI models.
Flathead side valve motors were still very popular during the 40's,
The displacement of the flatties ranged from 45", 74" to 80" but heat and oiling issues favored the OHV design.
Typically, police departments would order the E type motors, or 61Ci
This was both utilitarian and less expensive, but the 61s had a smaller lift cam that allowed the bikes
easier starting in hotter conditions. Stop and Go stuff...
Now, 1948 was an unusual year, because it was also the last year for the Spring Fork front end, but first year PANHEAD!!
Over zealous  engineers at the factory were always a little ahead with design, This was evident in 1965, when a frame change, electrical change, and design change pre-dated the
shovel head intro in '66.
And historically, you can see that the production guys were busy selling in stock, manufactured inventory. David Monahan, Harley expert and noted restorer owns a 1949 Spring Fork Panhead.
Sometimes the right hand doesn't want to know what the left hand is doing...Especially when there are paychecks involved.
In 49, the Hydra-Glide was introduced which, by definition, was the first hydraulic suspension bike produced by the factory.
Similarly, the Duo-Glide trademark was introduced in 1958 and had BOTH  front and rear suspension.
The Electra-Glide surfaced in 1965, 12 volt system and frame change were introduced.
Unfortunately, the Shovelhead design was only still on the drawing board.
Or, as I might guess, they simply wanted to use up the existing inventory of Panhead motors sitting around.