The common narrative is that Phoenix's spectacular growth was made possible by air conditioning. But that's only partly true.
Some of the hottest places in America were big cities in 1930, when air-conditioning units were large and expensive, confined to the largest buildings with money to spend. Among them were New Orleans (458,762), Dallas (269,475) Houston (292,352), and Atlanta (270,366). These cities suffered not only very hot, but also humid, summers. Phoenix, by contrast, had a population of only a little more than 48,000 that year. Even El Paso, the city that Phoenix leaders aspired to surpass as the business capital of the Southwest, held 102,421 people.
Before the beginning of the great post-war migration to the yet-to-be-named Sunbelt, the Intermountain West was lightly populated and a magical place unknown to most Americans outside of movies. The entire state of Arizona had a population of fewer than 436,000. The Intermountain West population was about 3.7 million out of a total U.S. population of 123 million. In other words, those seven states had fewer people than today's metropolitan Phoenix.
The great impediment to Phoenix's growth was not as much heat — note the cities above — as isolation. Cut off from the east and north by nearly impenetrable mountains, and from the west by forbidding desert, Phoenix was far from natural routes of commerce or travel. This began to change as branch lines of the Santa Fe and Southern Pacific railroads reached the emerging agricultural empire of the Salt River Valley in the late 19th century (the railroads' main lines had been built through northern and southern Arizona). This changed dramatically with the completion of SP northern main line through the city in 1926. Along with new highways, this forever broke through Phoenix's seclusion.