Which airline first used the advertising slogan "Fly the Friendly Skies" in its marketing campaigns?

Airlines have created some of the most memorable advertising slogans in transportation history. These taglines not only build brand recognition but often capture the essence of what the company wants customers to feel about their service. This question focuses on a groundbreaking slogan that helped transform an airline's brand perception during a critical period in its history. Test your knowledge of transportation marketing history!

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United Airlines
American Airlines
Pan American World Airways
Delta Air Lines