Excavation of the site for the Empire State Building began on January 22, 1930, and construction on the building itself started on March 17.

 The project involved 3400 workers, mostly European immigrants, along with hundreds of Mohawk nation iron workers; 14 of the workers died during construction.

The project was hurried to completion in order to take the title of "world's tallest building" from the nearby Chrysler Building.

The Empire State Building was officially opened on May 1, 1931, when President Herbert Hoover pressed a button in Washington, D.C. that turned on the building's lights, 410 days after construction commenced.

 http://en.wikipedia.org/wiki/Empire_State_Building

http://www.nypl.org/research/chss/spe/art/photo/hinex/empire/empire.html

About the reward I do not know , but the speed is also a function of the specifications: today buildings have to stand a 7 richter scale earthquake , strong winds ; I think that 75 years ago the standards were not so high. Any way - the speed it was built is really remarkable.