The US alone was able to produce much more than both Germany and Japan.
Germany was not able to produce as much, as their equipment was usually more stronger and durable, but it also meant it took longer to produce. They also continue to research options for a 'stronger weapon', and so their funds were little
Japan did not have the resources to produce good technology. Instead, they relied on their human strength and their beliefs.
The US, though their equipment was only average, were much more easier to produce, and combined with large funds, resources, and the populations' willingness to work, made their output larger than almost all the countries combined
