The Pentagon on Wednesday awarded a $4.3 billion contract to modernize its health-records system to a team led by Leidos Holdings Inc. and Cerner Corp., the biggest federal information-technology project since the troubled rollout of the HealthCare.gov insurance exchange in 2013.

The highly coveted project is expected to take seven years to transfer records of 9.6 million military personnel from a fragmented patchwork that still includes paper files to a single electronic system.

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