The 4000 lb High Capacity Mark I bomb – actual weight around – was a welded, cylindrical shell of thick steel. The body of the bomb was in diameter and long. The nose of the bomb was conical and a long lightweight, empty cylindrical tail with a closed end was fitted, for a total overall length of . A T-section steel beam was welded to the inner surface of the bomb to strengthen it. Subsequent Mark II and Mark III HC bombs differed in detail; the conical nose was replaced with a domed nose and the number of fuzes was increased from one to three to guarantee detonation. The Mark IV bomb did not have the T-section beam and the Mark V and Mark VI bombs were versions manufactured in the United States.