Final answer:
The Martin B-26 Marauder was the bomber effective in attacking bridges and transportation targets before D-Day, helping to weaken German infrastructure and logistics.
Step-by-step explanation:
The US bomber that was particularly effective in attacking bridges and transportation targets before D-Day was the Martin B-26 Marauder. These aircraft were known for their ability to deliver precise bombing runs on tactical targets such as bridges, rail lines, and supply routes that were critical to the German war effort in Europe. The B-26 Marauder's combination of speed, range, bomb load, and defensive armament made it a formidable bomber for such missions, contributing to the weakening of Germany's infrastructure and logistics prior to the Allied invasion of Normandy.