Lyric discussion by WeaselSpleen 

Seems fairly clear... This song presents the thoughts of a World War II airman who flew a bombing raid on Tokyo in the early stages of the war.

The metal dragon is an aircraft. The sun reflects off the plane's canopy. The plane is a B-25 bomber, thus the '25.

"Dark flak spiders" is a great description of the characteristic clouds of smoke produced by anti-aircraft fire.

Toy city streets is a reference to how the streets look from the air, and possibly also a bit of a pun on Tokyo.

Clumps of mushrooms are the bursts of the dropped bombs on the ground.

(Contrary to popular belief, mushroom clouds are not a distinct feature of nuclear explosions, but happen with conventional explosives as well, just on a smaller scale.)

In April 1942, a group of B-25 bombers took off from an aircraft carrier they were not designed for, on a mission to bombard the Japanase industrial infrastructure in Tokyo. The mission was dangerous, and some of the men did not return. A book entitled "30 Seconds Over Tokyo" was written about this mission, and was later made into a movie.

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