One of, if not the best book I have ever read is H.G. Wells War of the Worlds. The following passage is near the start of the book:

"Then slowly the hissing passed into a humming, into a long, loud, droning noise. Slowly a humped shape rose out of the pit, and the ghost of a beam of light seemed to flicker out from it. Forthwith flashes of actual flame, a bright glare leaping from one to another, sprang from the scattered group of men. It was as if some invisible jet impinged upon them and flashed into white flame. It was as if each man were suddenly and momentarily turned to fire."
In the first world war, if you had been in the German trenches near Mametz, and had braved the British artillery barrages to peer across no-mans land on the morning of July 1st, 1916, you would have seen something horrifying and similar to Well's words happening in front of you.

Like War of the Worlds the main driving force is the protagonists partner. William Howard Livens was the son of the Chief Engineer at Ruston Engineering Company. He followed in his fathers footsteps training to become a civil engineer. However on the day he finished his studies Britain declared war on Germany. Like many millions of others he immediately volunteered. With his training he was given a commission in the Royal Engineers.
Livens with his much more successful "Livens Projector"
In May 1915 Livens fiancée was in New York and she set sail for England, the ship she was to take was the ill fated RMS Lusitania.

Upon hearing of the fate of the liner Livens swore an oath of vengeance*. He began working on ways and means to get back at Germany. Luckily for Livens his fiancée hadn't been on board and he got the good news three days later, however this didn't temper his anger. Putting to work his inventive nature and previous home experiments on gas and flame weapons Livens soon got noticed. He proposed a new weapon of war, it became known as the Livens Large Gallery Flame Projector.

There is a clue in the name of how it was intended to be used. Gallery referred to tunnelling warfare, which was an extremely hazardous occupation. Nothing could survive above ground in no-mans land, so the obvious answer was to dig underground. In most cases the tunnels were used to place mines.
The most famous example is the Lochnagar mine, the largest man made explosion at the time, with its report heard in London. The tunnellers had to be completely quiet lest the Germans hear them. If they were heard then all manner of nasty counters could be employed, with the fate of being buried alive a very real possibility.

Livens proposed something new. A tunnel would be dug as near as possible to the German lines, and the Large Gallery Projector then installed in it. The Projector was 56 feet long and weighed two and a half tons. It used a diesel engine to power a pneumatic system. The pressure would force the head of the machine out of the ground, then squirt a stream of burning kerosene and diesel fuel over 60m. Then this cloud of flame could be worked across the Germans front lines. Due to the amount of fuel needed the Projector could only fire three shots of ten seconds each.
This monstrous machine had to be carried into the tunnels in pieces, once there it was assembled in place. All of this had to be done in utter nerve wracking silence. After it was complete the cans of fuel had to be manhandled into place.

On the 28th of June one of the four Projectors to be used in the upcoming Somme offensive was being carried down to the trench, when a German artillery barrage fell upon the men. Throwing the pieces about the soldiers dived for cover. After the barrage lifted the components were recovered and stored in the entrance to the tunnel, only for a later barrage to land a shell squarely on the tunnel entrance.

However despite this setback at least two, possibly three, Projectors were in in place for the 1st of July. On that morning the great British offensive of the Somme was launched. On the first day 60,000 soldiers were killed. To put that into perspective that's almost the same number as the current British Army strength. Even with these massive casualties the British barely made a dent in the German lines, apart from at Mametz.
The seven men operating each machine switched on their engines, opened the taps and activated the Projectors. On the surface the roar of the engine would have been muffled to a quiet droning, barely audible over the artillery barrages, and the detonation of several mines under strong points.
Slowly a square shape forced its way through the ground, and rose into the air. Forthwith a spark, then a boiling cloud of smoke and flame sprang from the Projector. This terrible jet washed upon the German trenches and doused them in flame.
The British assault reached the German trenches, and was successful. Its been argued that elsewhere in the battle of the Somme the same tactics were used to less effect, and that the only differences in the attack plan was the use of the Livens Large Gallery Projector. The limited employability and huge size meant that the Projector was only used once more in combat.
Livens went on to design the far more successful Livens Projector, which was used through out the rest of the war as the principle means of delivering gas or incendiary shells.

*Some sources say it was the casualties at the Second battle of Ypres that caused Livens to make this oath.