How about the DUKW?
An inspired, straightforward & incredibly useful device that assisted enormously in many of the great amphibious assaults (some naval officers feeling that Sicily simply would not have worked without them). Wrecked harbours bypassed, vital supplies taken straight to troops, ability to handle remarkably rough seas & even some weapon carrying capability, with a military lifespan that ran into the 70's and 80's.
All from a design process of c.3 months with another 42 days to build the first prototype.
Cheers,
Adam.