The Welsh Government has awarded the job to Costain, in partnership with Atkins, Halcrow and RPS Group, to design and construct an 8km section between Brynmawr and Gilwern.
The challenging scheme passes through a number of sites of special scientific interest including the Clydach Gorge within the Brecon Beacons National Park.
Costain won out from a shortlist including consortia containing Spanish firms – Bam Nuttall/Ferrovial and Dawnus/Obrascon Huarte Lain.
Carillion and a Morgan Sindall/Griffiths joint venture also lost out.
Costain chief executive Andrew Wyllie said: “We are delighted to have been awarded this contract and are committed to deliver a high quality, safe and cost effective scheme for the Welsh Government and the local community.
“This award demonstrates the strength of our position as one of the UK’s leading solution providers in the Highways sector.”