We know almost nothing about the actual playing characteristics of European flutes from before 1500. There are a number of written references to, and depictions of, transverse flutes in the 14th century, but little mention of of them in the 15th. No instruments from these periods are known to survive.
We find the first written descriptions of how to play flutes from the 16th century. And there are a number of surviving instruments from that period that can be studied, perhaps played to some extent, and measured and copied.
The renaissance flute, the type of instrument used in Europe from roughly 1500 to 1650 or so, can be admired for its sophisticated capabilities within a simple and basic form. The instrument has a narrow cylindrical bore with only an embouchure (mouth hole) and six small tone holes. It uses a somewhat different fingering system than that found on other six-hole flutes, which might otherwise appear to be similar. See Renaissance flutes.
One does not hear much about the transverse flute in the 17th century until near the end.
The flute was dramatically remodeled in the late 17th century to make it more suitable for playing the type of music that violins were playing. The new flute had a conical bore and one key for the little finger of the right hand. It was very different from the renaissance flute; as different, say, as the Boehm flute was from the simple system instruments of its time. See Baroque flutes.
While the one-key flute continued to be used throughout the late 18th and entire 19th century, makers began to add keys to some models, at first in England. The keys elimated the need for "forked fingerings" and made the flute more even. Changes in the bore (whether keys were used or not) produced brighter sounding flutes (particularly in Germany) and flutes with easier high notes (particularly in France), while bore changes plus larger tone holes and the use of keys produced louder flutes (particularly in England). These changes were evolutionary rather than dramatic or revolutionary. See Classical flutes.
The 19th century was a time of great differences in national styles. See 19C keyed flutes, III.
Theobald Boehm's first commercial model had "scientifically placed" holes, as well as a revolutionary new mechanism and fingering system. But it retained the conical bore and some of the sound of the ordinary flute. A huge change occured later when Boehm applied his mechanism to a cyclindrical tube with larger holes. See The Boehm flute in the 19th century. A number of professional players took up the Boehm flute in France, and the Boehm or another "modern flute" in England. But the Boehm flute was not common or popular for many years in Germany and other parts of Europe.
Experiments with the design of flutes and other instruments have always taken place, but these grew exponentially in the 19th century. There were numerous experiments before Boehm, and many more after his work. The primary goals of almost all of the innovators were flutes that were louder and more even in tone; facility was also of great importance.
The French took to the Boehm flute relatively early. The Germans mostly ignored it for several decades. The English were impressed with the sound but some disliked the fingering and used Boehm-influenced flutes incorporating some of the old fingering. See The Boehm flute in the 19th century. Many of the English "modern flutes" and "hybrid" systems are very clever and allow great facilty. They eventually lost out to the Boehm flute in part because of the complexity of their mechanism, not because of their sound. See English modified simple system, "hybrid", and "modern" flutes.
Some 19th century German players and conductors did not like the sound of the Boehm flute; they prefered the character and tonal flexibility of the conical bore (and many players did not want to change fingering). Late in the 19th century, highly mechanized conical models were introduced that were louder and more even, in order to compete with the Boehm flute. See Further development of the simple system; Reform flutes.