Hull suffered a major raid on the night 18th March 1941, one of several large raids that spring, although it had been frequently bombed since the summer of 1940 and would continue to be a target into 1943.The Hull Bombing Map has a thorough analysis.
The spring weather saw a the Luftwaffe bombing campaign renewed with vigour. Numerous targets around the country were now being hit, with the emphasis on port cities, as can be seen from the weekly Home Security Situation report:
The week was characterised by a series of heavy raids, mostly on a number of ports and on London. Many were widespread, although docks or industrial dwellings in specific areas received the greater proportion of the bombs. Twenty-one R.A.F. Stations or aerodromes were hit, but with only slight damage.
59. On the night the 13th/14th March Merseyside was attacked for the second night in succession, but less severely, Wallasey and Southport suffering most. Severe bombing took place at Hull, and there were minor incidents in all other regions, but the main damage was on Clydeside, where Glasgow and Clydebank suffered their first heavy raid of the war.
60. On the 14th/.15th March Clycleside again suffered most. There was widespread bombing in the West Riding, which, according to enemy broadcasts, was directed on Sheffield, but the most extensive damage and casualties in this area were at Leeds. Hull and Sunderland also suffered, and bombs were again dropped in every region. London was not heavily attacked. Plymouth and Portland also suffered short but intense raids.
61. On the 15th/16th March serious bombing was confined to South and South-East London and the Home Counties.
62. On the 16th/17th March Bristol suffered heavy damage and casualties, especially in the Central districts.
63. On the 17th/ 18th March slight damage was caused in the North Midlands and East Anglia.
64. On the 18th/T9th March: Hull suffered most, but other places in the \ East Riding were involved, noticeably Scarborough, which was bombed intermittently for four hours. This seems incongruous in comparison with the value of other objectives in the district. Some bombing took place in the North Midlands, Eastern Regions, London, the South and South-East Counties and Folkestone.
65. On the 19th/20th March : Bombing was concentrated on East London and the London Docks, where there were more major fires than on any date since the 29th December, causing considerable damage.
From the weekly Home Security Situation Report, 19th March 1941, see TNA 66/15/38.