Backed by an army of lawyers, Paul Slough, Evan Liberty, Dustin Heard and Nicholas Slatten appeared dressed in suits and ties before a federal court in Washington for the start of jury selection.
The trial was expected to last between five and six months, Slatten's lawyer Thomas Connolly told AFP.
Judge Royce Lamberth said an unspecified "very high number" of Iraqi witnesses is scheduled to testify about what they saw on September 16, 2007 in Baghdad's Nisour Square.
The Blackwater employees were guarding a US diplomatic convoy when they opened fire, killing 17 unarmed Iraqi civilians according to an Iraqi investigation, or 14, according to the US count. The hail of gunfire also wounded 18 people.
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The killing exacerbated Iraqi resentment toward Americans, and was seen by critics as an example of the impunity enjoyed by private security firms on the US payroll in Iraq.
Blackwater, whose license to work in Iraq was revoked by Baghdad, was renamed Xe Services in 2009 and then Academi in 2011.
Slatten is charged with the first degree murder of a civilian. Slough, Liberty and Heard are accused of voluntary manslaughter of the 13 other victims.
All four have pleaded not guilty.
Their trial follows a tortuous legal process that saw the case slowly wind through US courts.
In 2009, a US judge dismissed charges against five former Blackwater employees because certain statements they made immediately after the event could not be used against them.