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"At a time of unprecedented industry change, the Board concluded it is in the best interests of the company to combine the roles of Chair and CEO in order to drive the most efficient execution of our plan and vision for the future," said Theodore Solso, GM's outgoing Chairman, in a statement.
"With GM consistently delivering on its targets and on track to generate significant value for its shareholders, this is the right time for Mary to assume this role."
Barra became CEO of GM in January of 2014, after spending her entire career with the automaker.
She's the first woman to run a major car company.
And now she holds both the CEO and Chairman jobs, which is GM's tradition.
"The GM CEO is usually also the Chairman," said Jessica Caldwell, a senior analyst at Edmunds.com, a consumer auto and information resource.
"She was tested for a few years, and she passed," Caldwell added.
Barra was unanimously elected to the chairmanship by the GM board, the company said.