Technically, a cannon is a long-barreled gun that fires its shot at a near-horizontal trajectory; a mortar (usually short-barreled) fires at a high trajectory and a howitzer stands between these two. Gun-howitzers can vary their angle of fire between zero and about 60° - 70°.
The first gun-howitzer was the French "Canon obusier de 12" alias '12-pounder Napoleon', introduced in 1853 CE. It could fire a solid ball at a large angle up to 3,000 meters; at a small angle up to 1,500 meters; grape shot to about 600 meters or canister shot to 300 meters. The maximum rate of fire was about 4 rounds per minute. It was powerful enough to damage fortifications, yet light enough to be maneuvered around on the battlefield. Its performance was so good and its versatility so great, that it quickly replaced all earlier field guns. The Napoleon was the last major muzzle-loaded smoothbore field gun.
Gun-howitzers remain in use to this day, but muzzle-loaded smoothbore guns were soon replaced by breech-loaded rifled guns.