Volley gun
The Ribauldequin was a medieval version of the volley gun. It had its barrels set up in parallel. This early version was first employed during the Hundred Years' War by the army of Edward III of England, in 1339. Later on, the late Swiss army employed it.
History of gunpowder
The earliest possible employment of volley fire for firearms occurred in late-14th-century China during a military conflict between Ming and Mong Mao forces. Volley fire was also possibly implemented with firearms in 1414 during the Yongle Emperor's campaigns against the Mongols, and possibly again in another expedition in 1422. However the language used in these sources is unclear as to whether or not "repeating" fire was part of the technique implemented. For example, during the 1388 anti-insurrection war waged against the Mong Mao by the Ming general Mu Ying, the Ming troops equipped with guns and fire arrows were arrayed in three lines. The general Mu Ying explained this was so that "when the elephants advance, the front line of guns and arrows will shoot all at once. If they do not retreat, the next line will continue this. If they still do not retreat, then the third line will continue this." When the armored war elephants broke into a run, charging the Ming lines, the Ming forces stood their ground, "shooting arrows and stones, the noise shaking the mountains and vallies. The elephants shook with fear and ran." According to the "Ming Shilu", half the elephants were killed while 37 were captured, and of the 100,000 strong insurrection force, at least 30,000 were killed, and 10,000 were captured. Andrade and other historians have interpreted this passage as evidence of volley fire, however he admits that it is ambiguous as to whether or not the Ming lines practiced repeated fire and reloading, so at best it can only be considered a limited form of volley fire.