There had been two such accidents with skyscrapers in New York before hair-raising 9/11 attacks.

The first one happened at 9:40 am on Saturday, July 28, 1945. A B-25 Mitchell bomber, piloted in thick fog by Lieutenant Colonel William Franklin Smith, Jr., crashed into the north side of the Empire State Building, between the 79th and 80th floors.

The second one took place in 1946. On the evening of May 20, a United States Army Air Forces Beechcraft C-45F Expediter airplane crashed into the north side of 40 Wall St, as the visibility was poor because of the fog.

More Info: en.wikipedia.org