Technology is best left abstracted, as it tends to be a repetative and ends up with too much control and an unrealistic concept that technology wins wars.
The issue is, technology tends to be based upon need, rather than genius invention. Most developments were done by basis of realizing that your current technology is not useful. Country A develops weapon 1, Country B develops weapon 2 to counter weapon 1, Country A then develops their counter. This is why Germany started their campaign in Russia with PzKpfw.III, encountering T-34 (better than PzKpfw.III), and developed the PzKpfw.V (better than T-34), and so on.
Technology does not necessarily equate 'better'. Only a few weapons were 'revolutionary' or 'better' in regards to previous quality. Aircraft and Tanks were the primarily evolving technology piece through the war timeframe, as most other weaponry (Warships, Small Arms, Artillery) remained the same for decades. Indeed, some early war weaponry was actually of better quality than later war weaponry, which was developed in order to ease mass production (Thompson SMG vs M1 Grease Gun for example). Some additional weaponry was developed (such as improving man-portable anti-tank weaponry, radar for warships), but in reality, the average soldier was similarly equipped in 1935 as they were in 1945.
What is more important than technology was evolving strategy, combat experience, and unit composition. A German division of 1939 was different to that of 1944 primarily due to a decrease in size (removal of a few battalions to create new divisions), but also an increase of firepower, this resulted in a smaller, yet stronger division. The equipment employed by a 1944 German division was virtually the same as that of the 1939 division (in fact, many cases much older equipment).
Technology would be best placed outside of human control, responding to actual game situation rather than player desire. While winning, a nation tends to keep the same strategies, and same equipment, usually resulting in a slump of development. While losing, a nation devotes as many resources in order to counter enemy developments (British ASW, German Jets, etc.). In the end, it isn't the technology that makes or breaks armies (An allied division was just as powerful as a german division, albiet through different methods).
A lot of technology was done by individuals outside of government control, or, a government request was issued, and industries competed for design. The push and pull for technology was a greater factor than actually manifesting the development. All technology used in WW2 was technically attainable at any point, but when the tools you have now are completely effective, why waste resources developing new and flashy ones? This is a paradox in most technology based games, where the drive to research is purely based on no motive, other than to get the next best thing, even though, in reality, you probalby don't 'need' it.
Issues pressing technology...
1. Necessity (what you have is not working)
2. Level of Innovation (how open your nation is to new ideas)
3. Policy and Existing concepts (national strengths and weaknesses, Britain is more interested in Air Defense issues than a strong armoured force)
This is probably not what people want, but, in regards to reflecting a closer reality in the R&D aspect of military conquest (significantly less of an issue than production), it may be good to address it.