(The British and Europeans were actively fighting in the War since 1939 but the USA did not get really involved until January 1942. We mainly refer here to the period from 1940 onwards, as that is when the Chiefs and Scouts first got their famous skirted fenders.)
During WW II Indian made a de-tuned Sport Scout for the Allied armies called the 640, which embodied a 750cc (45ci) power plant. (Six refers to the model and 40 to the year of manufacture, e.g. a 341 is a 1941 Chief.)
Indian also made many model 741's which were 30.50ci (or 500cc). These were slow but very reliable. Many survive in Britain and New Zealand.
In civilian terminology, the smaller 500cc (30.50ci) models were called "thirty-fifties" or "Junior Scouts" or "Pony Scouts". In military terminology they were model 741b's.
During the war Indian made about 33,000 military cycles compared to about a reported 50,000 or 90,000 made by Harley. Harley not only sold more but their contract provided that they earned more per unit. Instead of profiting by the war like so many big manufacturing companies in the US, the Indian Motocycle Company made a loss!
Many 30.50ci and 45ci Scouts and some 74ci Chiefs were sold to the American, Canadian, Australian, New Zealand and British military during the war, and many of the smaller models remain in Europe and New Zealand.
As mentioned earlier, the larger military 750cc (45ci) versions (called 640b's) were de-tuned Sport Scouts. The US military versions had their engines stamped WLA while the Canadian ones, which were converted to hand clutch operation by Bernie Nicholson, were WLC.
A number of Chiefs were sold to the Canadian and US military and Australian army.
Indian must have sold some models to the French as Indian Chiefs are known to have existed in France, with the speedometers written in kilometres instead of miles.
The US President gave a large number of 80 CID Chiefs to the Shah of Iran in 1951, and most of these seem to have been bought and brought to North West Europe.
No Indian history is complete without mention of the advanced military 841 model. Sadly only 1000 were made before the US Army decided to order huge numbers of Jeeps instead. The 841 (and similar Harley XLA which suffered the same fate) copied the successful German BMW army motorcycles including shaft drive and 4 speed hand clutch/foot shift. (Actually the 841 looks more like a Moto-Guzzi because it was a 90 degree V instead of a 180 degree flat or "boxer" engine layout. The Guzzi actually copied the Indian because the Guzzi V twin did not come about until the late 1960's.)
Indian did not use these advanced bikes as postwar civilian models due to the financial burden in the post war period
The war years were not good financially for Indian, and during this period they had invested heavily in expensive development work. Designing the 841 transmission and drive shaft, rear hub and setting up the jigs and tooling was a great expense.
Also, under Ralph Rogers in the late 1940's, Indian spent millions designing and building a brand new series of modern singles and parallel twins. With the coffers empty, the probability of the 841 being further developed for the post was civilian market was short lived.
Indian did however make use the 841 fork in its 1946-48 Chiefs (with slight modification).
War Era Prototypes Models
There were some secret factory experimental fours built during the war. These are even rarer (and much faster) than the 841's. Opposite is a very rare photo of a 1941 prototype of an 800 c.c. OHV Four with shaft.
The Indian Four always had chain drive but was a natural for a shaft drive, being an inline automotive design.
A close look at the 1941 prototype photo shows the military 841 frame and shaft drive. The forks however are telescopic which did not appear in civilian production until 1950.
Also note the modernized four cylinder engines. (The pre-war fours were F head, i.e. inlet valve in head, side valve exhaust.)