In his first move as MD, Key decided to split the head coach role between red and white ball cricket and McCullum was initially thought to be a leading candidate to coach the white ball side. However, the 40-year-old has been appointed Test coach instead, agreeing a four-year deal.
According to a report from the i, McCullum did not apply for either role, with Key instead approaching him. It is also stated that Key made McCullum his 'first choice from the outset', with the New Zealander 'convinced he could be successful in the role'.
The report says the ECB scelection panel found McCullum's vision for the Test side 'compelling' and Sir Andrew Strauss, a member of the panel, told Sky Sports: "He blew us away with his clarity of thinking and his simple approach.
"He's a very positive guy with a very clear mindset and he will embed that in that Test team at a time when confidence is a bit low and people need a bit of clarity and direction. He's an impressive guy."
Speaking on his SENZ radio show 'Breakfast with Baz and Izzy', McCullum said he is excited by the challenge of turning around a side that has won just one of their last 17 Test matches. "I thought 'crikey I've got to give this a crack if they think I'm the right person'", he said.
"It all happened quickly but I'm incredibly humbled and delighted to be in the chair. If you are prepared to change your life for something it has got to be something a bit grunty, a bit meaty and that is what the challenge of trying to bring a team that is rock bottom out of that situation is.
"The white ball job didn't interest me as such because they are flying. They are one of the best teams in the world, they are the world champions after the 2019 World Cup.
"They've got Eoin Morgan in there who is a very good friend of mine who has built a structure and a system that will continue well after he has finished playing, so I wasn't interested in a cushy gig. I'm looking to try to build something long term, sustainable and successful, that is where the challenge lay, plus I love Test cricket."