The Germans did develop some helicopters before and during WWII, like the Focke-Wulf 61 (1 Person) (Twin transverse rotor system), the Flettner Fl 282 (1 person) and the Focke-Achgelis 223. Around 20 of each were built during WWII. There was an effort to produce more, but these efforts were frustrated by the bombing campaigns of the Allies and effort and material was needed elsewhere in the war effort. (1)

But apart from the Achgelis these did not have any real usable range or payload, and were not of much use other than to serve as artillery spotters. This role was very well filled by light aircraft such as the Fieseler Storch. In addition in Europe there were ample landing and takeoff possibilities near enough to the front to use light aircraft in the role that helicopters of the day would be able to fill. In addition these helicopters would be sitting ducks for the allied airforce fighters whom by 1943/44 had achieved air dominance over the battlefield.

The first Helicopter of any serious use would be the Sikorsky R-4 which was developed out of the Sikorsky VS-300 prototype for the US Army, and was ready for deployment by mid 1944. Around 400 of these have been produced and it did see action in Burma with the 1st air commando unit. It performed in the medevac and reconnaissance role, which was basically the role the familiar Bell-47 (MASH anyone?) would fulfill in Korea. In this theatre they were of added usefulness because light aircraft couldn't really operate in that environment due to lack of landing spots big enough. Though it's role was small it was promising, and therefor the helicopter as such was further developed in the US post WWII. (2) Furthermore the R-4 was of particular importance and more useful than the german designs because it utilized a tail rotor instead of dual counterrotating blades to counter torque. This new system was simpler, cheaper and above all lighter.

The main thing with helicopters is that they need a very outstanding power to weight ratio to have any usable payload. This was possible towards the end of the war with more efficient piston engines, but did not really kick off until the invention of the Turboshaft turbine engines found in today's helicopters. The UH-1 Iroquois (More commonly known as the Huey, first flight 1956, introduction to armed forces in 1959) was the first helicopter to utilize turbine engines and this was a massive improvement to the useful payload and flight envelope. And this Helicopter of course saw extensive service during the Vietnam war. From this platform the first attack Helicopter was developed: The AH-1 Cobra.

(1) http://www.warbirdsresourcegroup.org/LRG/fa223.html Donald, David, ed. The Complete Encyclopedia of World Aircraft. (2)http://www.centennialofflight.net/essay/Rotary/Sikorsky_VS300/HE8.htm