A shell or flued boiler is an early and relatively simple form of boiler used to make steam, usually for the purpose of driving a steam engine. The design marked a transitional stage in boiler development, between the early haystack boilers and the later multi-tube fire-tube boilers. A flued boiler is characterized by a large cylindrical boiler shell forming a tank of water, traversed by one or more large flues containing the furnace. These boilers appeared around the start of the 19th century and some forms remain in service today. Although mostly used for static steam plants, some were used in early steam vehicles, railway locomotives and ships.

Flued boilers were developed in an attempt to raise steam pressures and improve engine efficiency. Early haystack designs of Watt's day were mechanically weak and often presented an unsupported flat surface to the fire. Boiler explosions, usually beginning with failure of this firebox plate, were common. It was known that an arched structure was stronger than a flat plate and so a large circular flue tube was placed inside the boiler shell. The fire itself was on an iron grating placed across this flue, with a shallow ashpan beneath to collect the non-combustible residue. This had the additional advantage of wrapping the heating surface closely around the furnace, but that was a secondary benefit.

Although considered as low-pressure (perhaps 25 psi (1.7 atm)) today, this was regarded as high pressure compared to its predecessors. This increase in pressure was a major factor in making locomotives (i.e. small self-moving vehicles) such as Trevithick's into a practical proposition.

The simplest boiler for locomotives had a single straight flue. It was widely used by many of the early locomotive makers, including Blenkinsop's locomotives for the Middleton Railway and Stephenson's Locomotion No. 1.

This type of boiler is simple to manufacture and strong enough to support "high pressure" (for the period) steam with expansive working in the cylinders. There is also good gas flow through the large flue, so that the fire receives sufficient draught from the action of a tall chimney alone. However it also has little heating area, so is inefficient and burns a large amount of coal.

A simple flue must be long if it is to offer adequate heating area. In a short boiler shell, such as required for a steam locomotive, this may be done by using a U-shaped return flue that bends back on itself.

Richard Trevithick had already used a return flue with his first 1802 Coalbrookdale locomotive design and 1804 "Pen-y-Darren" engine. These boilers were heavily built of cast iron, short and flat-ended. His 1804/5 "Newcastle" locomotive (actually built in Gateshead) began to show one characteristic feature of the return-flued boiler, a prominent dome shape to resist steam pressure in the solid end opposite both furnace and chimney. In this case the boilermaking, now of wrought iron plates, was likely complicated by Trevithick's single long-travel horizontal cylinder (9 in × 36 in (230 mm × 910 mm) bore × stroke) which emerged through this domed end. This did make work easier for the fireman however, as he was no longer trying to reach a firedoor beneath the long crosshead of the piston.

William Hedley used this pattern of boiler for his 1813 locomotives Puffing Billy and Wylam Dilly. Through the Wylam colliery and its owner Christopher Blackett, Hedley would have been familiar with Trevithick's engine.