The board said Monday that fog rolled in a half-hour after the Cessna took off from a private airstrip near the small town of Waskada on Feb. 10, 2013.
The weather, combined with snow-covered ground, likely made it difficult for the pilot to see the horizon.
"There were few trees or other features to provide visual references," the board said in its report.
"In the absence of a visible horizon, the pilot likely experienced spatial disorientation, particularly if he initiated a turn to avoid the deteriorating weather."
The plane flew directly into a farmer's field. The report said the plane's wheels were not lowered for landing.
"Consequently, no aircraft technical malfunction was identified."
Darren Spence, 37, was an experienced crop duster who had taken his two sons and their friend on a pleasure flight that day.
The single father often took his boys, Logan, 9 and Gage, 10, up in the air on weekends. But it was a first-ever plane ride for their nine-year old pal, Dawson Pentecost.
At the time of the crash, Dawson's family said Dawson had phoned first to make sure it was OK to go.
Dawson's father was a volunteer firefighter and he later got a message on his pager that a plane had gone down in the area. He rushed out on snowmobile and discovered that no one had survived.
The board determined that Spence had completed all necessary pilot training and an annual questionnaire.
The questionnaire, aimed at updating pilots on changes in flying, had addressed the topic of whiteouts in previous years but not since 2008.
"In order for a pilot to escape from whiteout conditions, it is necessary to either effectively transition from visual to instrument flight or be able to quickly regain sight of visual contrast," said the board's report.
"It is generally considered to be a difficult task for even an experienced instrument pilot to make a successful transition from visual to instrument flight after inadvertent entry into instrument meteorological conditions."