World War 1, 1914 - 1918.
This is one of the most interesting periods of human history. The aftermath of the Industrial Revolution had already been allowing constant development of better steel and more advanced machinery for about a century, but things REALLY got moving as most of Europe suddenly went nuts and every military power involved raced to come up with new, modern ways to destroy each other.
This is most notable in the development of the aeroplane. At the start of the war, flying machines were little more than cheap, experimental novelties that, with luck, could fly maybe half a mile. However, the potential advantages of aerial warfare soon captivated the creative minds of war entrepreneurs, and with the eagerness of military science, airplanes were soon becoming more practical and reliable.
In 1919, pilots John Alcock and Arthur Whitten Brown made the first successful flight across the Atlantic in a bomber plane left over from the war, paving the way for regular passenger services.
(Click the link for a picture of the type of plane they used
http://upload.wikimedia.org/wikipedia/commons/e/ea/Vickers_Vimy.jpg
What really facsinates me, though, is the way new weapons and gadgets would often have to be put into production as quickly as possible, before the enemy could come up with something similar. This often meant that new inventions would be put to use on the battlefield long before anyone gave a thought as to how well they would actually work.
For example, the Germans came up with a new type of tank (the ONLY German WW1 tank that made it into production!) that got stuck in almost any kind of terrain and usually ended up lagging behind the infantry troops it was meant to assist.
Looked awesome, though. Click the link for a picture of a replica:
http://en.wikipedia.org/wiki/File:A7v-wotan-diorama.jpg
I should also note that the famous Stahlhelm (Steel Helmet), later known worldwide as the Nazi helmet, was first introduced in 1916 as the result of a similarily hurried design process. (The type of helmet they had been using up to this point, had been designed some time before the introduction of weaponry that could reliably penetrate hardened leather!)
But my favorite WW1 weapon (at least that I know about so far), has to be the 1916 Lebel bayonet. By order of the French government, the Remington company fired up their state-of-the-art machinery, employing their premium quality materials and top-notch craftsmanship skills...
...to produce 200 000 SPIKES for use by the French army! (Click link for picture.)
http://www.quanonline.com/military/military_reference/misc/photos/lebel_bayonet.JPG