Why U.S. wartime development had room to run

Did only the United States have the luxury of developing new equipment while also producing at wartime scale?

The United States had the clearest advantage because it combined resources, physical space, and safety from bombing; Britain managed some parallel work through the empire, while Japan and Germany had less room and more duplication.

American space, resources, and freedom from bombing allowed production and development to proceed together. Britain retained some capacity by distributing research across the empire, while Japan and Germany faced tighter material and operational constraints. Japanese Army-Navy duplication made scarce engineering work more expensive by forcing separate organizations to relearn similar lessons.