It was based on comments to the BBC Radio 4 Today programme by Dr Martin Griffiths, lead surgeon at Barts Health NHS Trust, making a comparison to Camp Bastion in Afghanistan.
He told the Today programme: "Some of my military colleagues have described their practice here as similar to being at Bastion."
The comments came just weeks before Mr Trump is due to make his first visit to the UK as president.
Last year he reportedly suggested there were "no-go areas" in London because of Islamic extremists.
Mr Trump also said guns would have put a stop to the Paris terror attacks of 2015, saying: "I mean, they never mention that. But they died in a restaurant and various other close proximity places. They were brutally killed by a small group of terrorists that had guns. They took their time and gunned them down one by one.
Simulating the attack, as the crowd laughed and cheered, he said: "Boom! Come over here. Boom, come here. Boom. If you were in those rooms, one of those people and the survivors said it just lasted forever."
He went on: "But if one employee or just one patron had a gun or if one person in this room had been there with a gun aimed at the opposite direction, the terrorists would have fled or been shot, and it would have been a whole different story. I mean, right?"
Mr Trump threw his full weight behind the NRA after briefly straying from the lobby groups's stance against gun control in the wake of the Parkland school shooting.
He said Democrats wanted to "outlaw guns" but the US might as well ban vans and trucks because they were the new form of death for "maniac terrorists".
"It seems that if we're going to outlaw guns, like so many people want to do.... then we are going to have to outlaw, immediately, all vans and all trucks, which are now the new form of death for the maniac terrorists."
Mr Trump basked in the adoration of 8,000 people packed into a convention centre in Dallas.
Amid chants of "USA, USA" the president said: "Your Second Amendment rights are under siege. But they will never, ever be under siege as long as I'm your president. We love you folks, we love you."
Mr Trump also thanked Kanye West, the rapper, for his public support.
He said: "Kanye West must have some power because, you probably saw, I doubled my African-American poll numbers. We went from 11 to 22 in one week. Thank you Kanye. Even the pollsters thought there must be a mistake."
It was the first NRA annual meeting since America's worst high school shooting on Feb 14. Seventeen people were killed by gunman Nikolas Cruz at Marjory Stoneman Douglas high school in Parkland, Florida.
Survivors of the attack have since led a movement calling for tighter gun control laws, and hundreds of thousands of people attended a march in Washington on March 24.
In the immediate wake of the Parkland shooting Mr Trump indicated he was open to stronger gun control measures including raising the minimum buying age from 18 to 21.
However, the White House and Congress have since taken no major steps toward tighter gun control.
Mr Trump's decision to address the NRA marked a political shift as Republicans head into mid-term elections in November, when many will receive funding from the gun rights group.
The president told NRA members in Dallas: "We cannot get complacent, we have to win the mid-terms. You watch how well we do in 2018, you watch. We've got to get Republicans elected."
