The first bombing of Tokyo was when the Doolittle Raid occurred on April 18, 1942.  This was when 16 B-25 Mitchells were were launched from the USS Hornet to attack Yokohama and Tokyo.  After they attacked they planned to fly into airfields in China.  There were also attacks from B-29s and they were flying at altitudes of 7,000 feet.  They were working to attack Tokyo, Nagoya, Osaka and Kobe.  They were dropping bombs from around 30,000 feet.  

There was one thing called a "fire bomb" and the first one of them was dropped on Kobe on February 3.  The first actual raid on Tokyo was on the night of February 23-24.  They destroyed around one square mile of the city.  The amount of bombs dropped was about 1,700 tons.  Of all the four cities that they attacked they destroyed nearly 31 square miles of cities. 

http://www.japan-101.com/history/bombing_of_tokyo.htm