As the war progressed, most nations developed self-propelled artillery. Some early attempts were often no more than a field gun or anti-tank gun mounted on a truck—a technique known in the British Army as carrying "portee". These were mobile, but lacked protection for the crew. The next step was to mount the guns on a tracked chassis (often that of an obsolete or superseded tank) and provide an armoured superstructure to protect the gun and its crew. Many of the early designs were improvised and the lessons learned led to better designs later in the war. For example, the first British design, "Bishop", carried the 25 pdr gun-howitzer, but in a mounting that severely limited the gun's performance. It was replaced by the more effective Sexton.