The first completely automated Bolo, designed to operate normally without a man aboard, was the landmark Mark XV Model M, originally dubbed Resartus for obscure reasons, but later officially named Stupendous. This model, first commissioned in the twenty-fifth century, was widely used throughout the Eastern Arm during the Era of Expansion and remained in service on remote worlds for over two centuries, acquiring many improvements in detail along the way while remaining basically unchanged, though increasing sophistication of circuitry and weapons vastly upgraded its effectiveness. The Bolo Horrendous Model R, of 2807 was the culmination of this phase of Bolo development, though older models lingered on in the active service of minor powers for centuries.

Thereafter the development of the Mark XVI-XIX consisted largely in furthur refinement and improvement in detail of the Mark XV. Provision continued to be made for a human occupant, now as a passenger rather than an operator, usually an officer who wished to observe the action at first hand. Of course, these machines normally went into action under the guidance of individually prepared computer programs, while military regulations continued to require installation of devices for halting or even self-destructing the machine at any time. This latter feature was mainly intended to prevent capture and hostile use of the great machine by an enemy. It was at this time that the first-line Bolos in Terran service were organized into a brigade, known as the Dinochrome Brigade, and deployed as a strategic unit. Tactically the regiment was the basic Bolo unit.

The always-present though perhaps unlikely possibility of capture and use of a Bolo by an enemy was a constant source of anxiety to military leaders and, in time, gave rise to the next and final major advance in Bolo technology: the self-directing (and, quite incidentally, self-aware) Mark XX Model B Bolo Tremendous. At this time it was customary to designate each individual unit by a three-letter group indicating hull style, power unit, and main armament. This gave rise to the custom of forming a nickname from the letters, such as “Johnny” from JNY, adding to the tendency to anthropomorphize the great fighting machines.

The Mark XX was at first greeted with little enthusiasm by the High Command, who now professed to believe that an unguided-by-operator Bolo would potentially be capable of running amok and wreaking destruction on its owners. Many observers have speculated by hindsight that a more candid objection would have been that the legitimate area of command function was about to be invaded by mere machinery. Machinery the Bolos were, but never mere.