Which famous WWII military product launch was initially kept secret from the American public, but later became the focus of a massive brand recognition campaign featuring the slogan "It's a military secret"?

Throughout military history, certain product launches have revolutionized warfare and shifted the balance of power. These technological innovations often represented years of research and development, sometimes conducted in utmost secrecy. This trivia question tests your knowledge about one of the most significant military product unveilings that dramatically impacted World War II strategy and national security messaging.

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The P-51 Mustang fighter aircraft
The M4 Sherman tank
The atomic bomb
The Jeep (Willys MB)