Two New Hampshire firefighters driving across the country face an uncertain mission when they arrive in California early this week to help battle the largest wildfire in California’s history, the Mendocino Complex Fire.

“A complex fire means there are multiple fires under that one complex so we’re not sure where they’re going to send us or what we’re going to be doing until we check in,’’ said Forest Fire Patrolman David Kullgren of Francestown, N.H., in a phone interview from the road.

Kullgren and Forest Ranger Tom Trask, of Conway, N.H., both part of the state’s Forest Protection Bureau in the Division of Forest and Lands, set out Friday morning in a brush truck, which carries a 150-gallon tank, pumps, chainsaw, tools, and hoses.

They were crossing Wyoming at around noon EDT Sunday, planning to arrive in Utah that evening, and by Monday at the Incident Command Post in Ukiah, Calif., where they will receive their assignment. Kullgren expects they will work 10 to 16 hours a day, carrying 40 pounds of gear in grueling conditions.

“With the engine, they’ll probably be in the backwoods or in the back mountains on smaller dirt roads in the areas larger engines can’t get to,’’ said Chief Steven Sherman with the New Hampshire Forest Protection Bureau in a phone interview Sunday. The two men will work for three weeks before returning to New Hampshire by plane, potentially leaving the truck behind for two more firefighters to take their place.

The differences between wildfires out west and structure fires are stark, he said. “A house fire, we call them a fire in a box. To get out of the box you just step outside,’’ Kullgren said. “In a wildfire situation you can’t just step away from it. It’s all around you.’’

This is Kullgren’s second time battling wildfires in northern California and the veteran firefighter’s 18th assignment out west over the past 18 years. It will be Trask’s first time, although he recently fought wildfires in Quebec. Kullgren said Trask was excited to “see big fire, see big country’’ and called it a great training experience for him.

It’s a job neither man takes lightly.

“When I was out here in 2015 I was burned severely on my lower right leg,’’ Kullgren said. “So, there’s a bunch of hesitation for that for me to go out here again...But, that’s what firemen do. We have an overwhelming need to help people.’’

Cristela Guerra can be reached at cristela.guerra@globe.com. Follow her on Twitter @CristelaGuerra.