What was the primary reason for the post-World War II economic boom in the United States?

The economic boom following World War II was the result of pent-up demand.  During the war, most production was directed to the war, weapons and ammunition, uniforms for soldiers, vehicles to move troops. For example, no vehicles for private consumption were produced during the entire war.  This meant that after the war was over, production of consumer products could resume, and people were more than ready to buy cars, appliances, and clothing.  At the same time, soldiers returning from war were getting married, settling down, and having children.  This meant there was a need for new housing. There was a dreadful housing shortage during the war, and I know that my own parents were thrilled to find a studio apartment to start their marriage.  Various programs for benefits helped returning soldiers to purchase their homes, and educational benefits allowed them to go to college.  This generation, having fought a war and done without for so long, was ready to consume!

Why did the US economy grow post-World War II?

I would say that the 1950s brought with it unprecedented economic progress because the war generated enough production as the wheels of industry moved after a long silence.  The Great Depression had caused industry to shut down and be invisible.  Yet, the need for war production and consumption helped to restart this process, causing a high level of material prosperity in America following the war.  At the same time, the United States was the unquestioned world economic leader.  Europe had been decimated by World War II, and the new nations that emerged from colonization were fledgling, at best.  Japan was crippled following the dropping of the atomic bombs, and China was not a world power.  The Soviet Union was seen as a militaristic adversary to the United States, but one that could not rival its wealth and material prosperity.  In the end, this world status also contributed to the expansion of the economy because there were new markets to be conquered internationally.  The market for exports was unprecedented as every nation sought to "buy American," for its own nation could not produce what was needed.  These factors led to the United States' economy expanding so quickly in the period after the Second World War.