Sociological Images has a great set of photos that show how factories camouflaged themselves during World War II. Especially interesting is this explanation they picked up from The Library of Congress World War II Companion:
A year before the United States entered the war, Kansas City’s Art Institute offered the country’s first classes in industrial camouflage, and other schools soon followed suit. In a short-lived blackout measure, steel mills in Gary, Indiana, were shrouded in thick smoke to hide their location from enemy places. The gold dome of the Massachusetts state house in Boston was painted gray, so it would not stand out, and elsewhere other important secular structures were topped with church steeples… The most ambitious deceptions, which fooled even local pilots, were the fake suburban neighborhoods and small towns built of plywood and chicken wire atop aircraft factories one the West Coast… Small plywood houses, rubber cars, clotheslines, and artificial plants dotted the three-dimensional landscape (p. 179).