Why was the F-104 called a "manned missile"?

The Starfighter's spectacular looks and performance gave rise to the nickname "the missile with a man in it." The formula sought was the smallest fuselage and wings that could be built around the most powerful engine available.

Among the design's background are interviews with fighter pilots from the Korean War in 1951. The intent was to break the trend toward ever larger and more complex fighters and obtain a simple, lightweight aircraft with maximum ceiling and climb performance. Greater speed, altitude, and climb rate were pursued even at the cost of maneuverability, armament, range, ease of maintenance, and onboard comfort. The goal was a lightweight day interceptor with rapid climb, high speed, and high altitude.

It was the first production aircraft to reach Mach 2 and the first to reach 100,000 ft (30,000 m) taking off under its own power. The wing's leading edges were so thin —0.016 in (0.41 mm)— that they posed a hazard to ground crew. The airframe was all-metal, mostly duralumin, with some stainless steel and titanium parts.