@roger,
"...The first known printed use of donut was in Peck's Bad Boy and his Pa by George W. Peck, published in 1900, in which a character is quoted as saying, "Pa said he guessed he hadn't got much appetite, and he would just drink a cup of coffee and eat a donut."[7] The donut spelling also showed up in a Los Angeles Times article dated August 10, 1929... The interchangeability of the two spellings can be found in a series of "National Donut Week" articles in The New York Times that covered the 1939 World's Fair. In four articles beginning October 9, two mention the donut spelling. Dunkin' Donuts, which was founded in 1948 under the name Open Kettle (Quincy, Massachusetts), is the oldest surviving company to use the donut variation..."
http://en.wikipedia.org/wiki/Doughnut