A boiler is provided that has an elongate base header, an elongate dome header, and a plurality of separate watertubes each having an intermediate section and opposite end sections. An upper one of the end sections of each watertube is connected to the dome header and a lower one of the end sections of each watertube is connected to the base header such that the intermediate section extends through a combustion chamber of the boiler. The intermediate section of each watertube has a substantially constant outer diameter along its full length and is closely spaced to adjacent watertubes within the combustion chamber. At least one of the end sections of each watertube has a transition that reduces the diameter of the watertube as it extends from the intermediate section to an outwardly-extending circumferential flange. This arrangement permits close spacing of the connection sites of the watertubes to the header and permits the connection sites to be provided in a linear array. The structure of a watertube, an assembly of a watertube and header, and a method of assembling a boiler are also disclosed.