It seems like many early machines were less carefully designed as much as they were put together, and modern machines like a plane are substantially more complex than early planes. However design software and systems have also improved as well as an increase in trained engineers and specialized design project teams. So how long would say an engine (Steam, diesel or other) take to design in say 1800 1850 1900 1950 or 1995?
Machinery of the past was hardly "just put together". Careful thought and planning went into the development of new designs in the past just as now. It's just a lot easier to design a steam or diesel engine now because we have so many examples of working engines to start from. A body of knowledge has been built up over time regarding what improves the function.
One aspect that has to be taken into account is that the ability to construct newer machines has depended on the ability to make the machine tooling for their construction, in a circular pattern of improvement.
For example, the pistons of Newcomen steam engine required hand filing and scraping to produce, and the standard for clearance was that an English penny would just barely fit between the piston and the cylinder wall. This is terrible by modern standard (which are roughly a thousand times better), but it would not have been practical to bore out the cylinders without a source of power.
Similarly, the first screws were hand filed, but hand made screws were applied to make the screw cutting machines which allowed more precise machine cut screws and standardization of threading.
Design in the past started with sketches and developed into detailed drawings. Computers were not used for drawings until after the late 1960s, and even then is was far from universal. Computers were used sometimes to analyze design work before then.