Jeremy Hunt says he would boost defence spending by £15bn over the next five years if he becomes prime minister.
The Tory leadership candidate's promise would mean spending on defence would rise to 2.5% of GDP by 2023/24, from its current 2%.
He said the move would help combat "new threats to western values" and show the UK is "ready to defend its interests".
Defence Secretary Penny Mordaunt, who backs Mr Hunt, said the government must give the armed forces "what they need."
"Jeremy's clear commitment to do that is one reason why he has my support," she added.
Sources close to Mr Hunt suggested the policy would be funded through economic growth and fiscal "headroom" set aside by Chancellor Phillip Hammond.
"I was the person who secured a historic funding boost for the NHS and as prime minister I'll do the same for defence," the former health secretary said.
"My plan for defence will give our brave troops the backing they need and show the world that when it comes to the new threats to Western values, Britain is back and Britain's voice will be strong," he added.
Mr Hunt's move comes after repeated complaints from US President Donald Trump over the defence spending of Nato allies.
The UK is one of the few European members to reach the current target of 2% of GDP.
while other Nato allies spent between 1% and 2% and has called for the UK to consider "decisively" increasing military spending after Brexit.
His pledge comes after .
There has not been a full-scale Strategic Defence and Security Review looking at future defence challenges and capabilities, since 2015 and one is expected in 2020.