The extra recruits will join the company's mobile workforce, where they will be responsible for reacting to changes in service requirements from communications providers and tackling peaks in demand for fibre broadband around the country.
Openreach is liaising with The Career Transition Partnership (CTP), an arrangement between the Ministry of Defence (MoD) and Right Management, to help facilitate recruitment of the new engineers. The CTP provides career guidance along with training and employment support to those leaving the armed forces.
This latest recruitment drive brings the number of new BT engineer hires this year to over 1,000 people.
"This is good news not only for the 400 people who secure one of the new jobs but for the whole of the country," said Jeremy Hunt, Secretary of State for Culture, Media, Olympics and Sport.
"BT is creating jobs while rolling-out the digital infrastructure UK businesses need to grow. Digital technology is fundamental to the way we live our lives and the way we do business, which is why we want the UK to have the best and the fastest superfast broadband in Europe by 2015."
In addition, Openreach is retraining and reskilling hundreds of existing staff so that they become engineers dedicated to the provisioning of broadband services for customers across the UK.
, an ex-BT executive, telling a parliamentary committee that BT was anxious about the number of engineers it had available to it - a claim which BT denied at the time.
"I know people in BT who are concerned about the capacity problem within BT to deliver [its exchange upgrade programme]. There aren't enough people on the ground to face this problem," said Mitchell at the time.
Between March 2011 and December 2012 Openreach expects to have recruited 1,650 new engineers, including the 400 announced today.
The UK government has said that it hopes to have the best and fastest broadband network in Europe by 2015, and has committed a minimum of £730 million up until 2015 to support the rollout. The money is being distributed to local authorities that bid for funding via a framework created by governing body Broadband Delivery UK (BDUK).