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

The transportation industry has given us some memorable advertising slogans over the decades, many of which have become embedded in popular culture. Behind these catchy phrases are fascinating stories of brand positioning and market competition. This trivia question challenges you to identify which iconic transportation company first used a particularly influential slogan that helped reshape its brand image and market position in a critical period of its history.

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United Airlines
Pan American World Airways (Pan Am)
American Airlines
Delta Air Lines