Could Germany have fielded true submarines earlier?

Could Germany have built modern underwater-optimized submarines instead of submersibles early in the Second World War?

A Type XXI-like arrangement was technically conceivable, but earlier batteries would give poorer performance, large boats would be fewer in number, and Britain and France could adapt.

The R class demonstrated during the First World War that a submarine could be shaped for submerged rather than surface performance. Large hulls capable of carrying extensive batteries also existed, so a Type XXI-like arrangement was conceptually possible by the early 1930s. Contemporary battery technology would still give it much poorer speed and endurance than the late-war boat, while a snorkel mattered less before centimetric radar made surfaced charging so dangerous. Building a few large advanced submarines would also reduce the number available at the start of the war and alert Britain and France to the new threat. Earlier Allied escort orders and an R-class-derived hunter-killer submarine could therefore accompany the German innovation rather than leaving it unanswered.