It depends on how far you want to go back and what period you refer to as Proto-Germanic. Germanic tribes went through what they called the Volkerwanderung, or, Migration Period and this came in successive waves over hundreds of years. Germanic people are, of course, Indo-European people and there are many theories about where they came from. Many say off the steppes of Eurasia as originally a nomadic, pastoral people. Many Proto-Indo-European words back that up. Hence, the horse and the chariot. If that is the case, their original bows were probably composite bows of steppe nomadic peoples. Over time, as the chariot was abandoned for all but ceremonial use, they would have gone to longbows as they were foot-fighters after settling. At that time, they tended to adopt shield-wall, spear, and sword tactics. However, bows were used often as well. Just my opinion, but the bows were probably of a flatbow design, though perhaps a D type longbow could have been used.
One can look towards the Scandinavian God Ullr and the Goddess Skadi and see that both of these deities were related to archery with Ullr specifically a God of archery and bowmen and Skadi a Goddess of bowmen and hunters. Lore related to Them may reveal some valuable clues.
The Elder Futhark Rune Eihwaz means "yew" but also specifically a bow made from yew. This gives us a valuable clue as to what wood was preferred for bowmaking. You can look up various translations of the Rune Poems and see references made to yew bows in talking about Eihwaz. So, again, there we have the critical clue that tells us what the preferred wood was for making the bows of the period. Some also say the Rune Elhaz (or Algiz as some know it) relates to yew as well and also to yew bows. These Runes predate the development of the English longbow for quite some time.
In addition, there is the Anglo-Frisian Rune Yr which is specifically "yew bow". So, again, yew is still the preferred wood for bows when the Anglo Saxons went over to England. Again, this Rune predates the development of the English longbow for quite some time.
Also mentioned as bowmaking wood of the period is elm. This would have been their secondary wood. It is interesting to note that the Cosmic World Tree in Germanic myth is Yggdrasil which, while it is debated, is thought to be a Yew by many scholars. Also, in Germanic myth, First Man is made from an Ash Tree while First Woman is made from an Elm. Ash is what was used for spearshafts, Elm was used for some bowmaking, while Yew made the best bows. Whenever trees and wood is mentioned in any ancient myth, that's where you'll find the crucial clues as to what was used for weapons, both for hunting and for war. One's life depended on the weapons, thus, the wood had to have had some Divine origin or link.
