Back shortly after 1900 a fellow named Conrad Hubert who lived in New York city realized this thing called a flashlight had potential to be very popular with home owners and house wives who lived in cities with inconsistant power grids. He also knew folks beyond the grid would like them as well.
By 1906 the carbon filament bulb had been replaced by a tunston filament. Bulbs were able to burn longer than a few seconds at a time.
A former associate of Mr. Hubert, Charles F Burgess had tweaked chemistries inside of batteries and had produced major advances in runtime.
Manufacturing numbers were increasing and flashlights were no longer seen as a novelty.
Now the idea that may have had the biggest impact on sales was when Mr. Hubert donated a few dozen of his Ever Ready (later changed to Eveready in the US) vulcanite 2 D lights to local police departments. They were a huge hit with cops and their wives.
A Franco made 2C made a few years later, was very similar to the 2D Ever Ready mentioned.
Once the flashlight was seen as a tool for crime fighting there was no end in sight.
I will add when it was deemed useful for military use when I know for certain, but by World War 2 a number of companies were making the TL-122A. Due to metal shortages the B was made of plastic and was widely issued to Allied troops.
Left is the US paratrooper A.
Right is the B that was largely distributed to British and American troops as well as other nations in the Allied campaign.
It has been said the TL122A was used near the end of WW1.
A bit off track, but relevant to the future of the industry:
In the 1920's an allegiance was formed to reduce the lifespan of light bulbs. Eventually it was disbanded but precidence had been set.
My first LED lights boasted claims of 100,000 hours user life. Then I noticed many were saying 50,000 hours. Still sounds great, right? But now, if you take notice there are some stating 30,000 hours. Some would say "it's due to over drive." Pish-posh I say...it's 'planned obsolecence repeating imo.
http://spectrum.ieee.org/geek-life/history/the-great-lightbulb-conspiracy
^^ The story of the light bulb conspiracy.
You decide.
Yeah the flashlight industry is
that big.