The Ordnance BL 15-inch howitzer was developed by the Coventry Ordnance Works late in 1914 in response to the success of its design of the 9.2-inch siege howitzer.

The howitzer was cumbersome to deploy, since it was transported in several sections by giant Foster-Daimler tractors.

The weapon was operated by Royal Marine Artillery detachments of the Naval Brigade, with one gun per battery. One gun was sent to Gallipoli but not used there. They were later transferred to the British Army. It was used at the Battle of the Somme in September 1916 and at the Battle of Passchendaele, also known as the Third Battle of Ypres, in October 1917.

It operated successfully where it was needed to destroy deep fortifications on the Western Front, but was limited by its relatively short range compared to other modern siege howitzers. The size and weight made it difficult to move and emplace. No further development occurred after the first batch of twelve, and instead Britain continued to develop and produce the 12-inch howitzer and 12-inch railway howitzer.

• Rolling shell along rails, Englebelmer Wood, Battle of the Somme, September 1916
• German soldier posing with unexploded shell
• Shell at Royal Artillery Museum, Woolwich, London
• "Granny" in action during the Third Battle of Ypres, 4 October 1917
• Howitzer inspected by a British delegation in 1916
• A 15-inch Howitzer being set up

• List of siege artillery