Donald Trump pledged to massively increase US military spending and swiftly demand a defeat plan of the Islamic State (IS) group if he becomes president, burnishing his defense bona fides Wednesday ahead of a forum with Hillary Clinton.

Speaking in Philadelphia, the Republican presidential candidate told supporters that he would ask his generals within the first 30 days of his presidency to craft a roadmap to the IS group's annihilation.

The military has become "so depleted," he said, that it requires a large-scale build-up, including troop increases and more aircrafts and ships.

Taking a jab at his Democratic rival ahead of their first joint event -- speaking back to back at a Wednesday evening forum on the president's role as commander-in-chief -- Trump called the former secretary of state "trigger happy and very unstable."

"Hillary Clinton's legacy in Iraq, Libya, Syria has produced only turmoil and suffering and death," he told the audience.

"Immediately after taking office, I will ask my generals to present to me a plan within 30 days to defeat and destroy ISIS," Trump said, using an alternate acronym for the IS group.

"This will require military warfare, but also cyber warfare, financial warfare, and ideological warfare," he said.

Providing more nuanced detail than in the broad-brush rallies to which his supporters have become accustomed, Trump outlined proposals for an active army of around 540,000 troops, an air force of at least 1,200 fighter aircraft, a 36-battalion marine corps and a navy of 350 surface ships and submarines.

"I will ask Congress to fully offset the cost of increased military spending. In the process, we will make government leaner and more responsive to the public," Trump pledged.

"We will be defended because without defense, we don't have a country."

- Winning with military voters -

Most national polls show a tight race with Clinton in the lead as the US presidential race enters its home stretch with just nine weeks until the November 8 election.

However, Trump is ahead by a wide margin of 19 percentage points among military and veteran voters, according to the latest NBC News/SurveyMonkey Weekly Election Tracking poll.

His 55 to 36 percent lead with the group comes despite recent remarks against the parents of a Muslim-American soldier killed in Iraq, members of the so-called Gold Star families who have lost loved ones in military service.

Trump also angered many in the military community with mocking remarks against US Senator and former prisoner of war John McCain for being captured in Vietnam.

Speaking at different times during their Wednesday evening forum, Clinton and Trump are set to answer questions about national security, military affairs and veteran issues in front of mostly veterans and active-duty service members.

Although Trump edged ahead of Clinton Tuesday in a new CNN/ORC poll -- by 45 percent to 43 percent among likely voters -- Clinton remains ahead in a number of other surveys.

An NBC News poll of registered voters shows Clinton's lead holding at six percentage points -- 48 percent to 42 percent.

Another poll, a 50-state survey by The Washington Post, showed Clinton with a solid lead in electoral college votes -- those that ultimately determine who wins the presidential vote -- even in some traditional Republican strongholds.