Part of the standard series, they were in most respects copies of the Pennsylvania-class battleships that immediately preceded them, carrying over the same main battery arrangement of twelve 14-inch (356 mm) guns, but now increased to 50-caliber.

Like the other standard-type battleships, they had a top speed of 21 knots (39 km/h; 24 mph) that allowed the fleet to operate as a tactically homogeneous unit.

Following the Japanese attack on Pearl Harbor in December, they were quickly transferred back to the Pacific, though they spent most of 1942 escorting convoys off the west coast of the United States.

New Mexico and Idaho were quickly decommissioned and sold for scrap, but Mississippi remained in service, having been converted into a gunnery testing and training ship.

Strauss accordingly suggested that the designers at the Bureau of Construction and Repair, under Chief Constructor Richard Watt, incorporate independent slides.

The General Board preferred the larger of the two, citing the heavier armor as needed to defeat the latest 15-inch guns being adopted in foreign battleships and the stronger secondary battery, which they viewed as necessary to ward off long-range torpedo attacks by destroyers.

Instead, he ordered the Navy to accept a repeat of the preceding Pennsylvania class, which might be fitted with separate slides for the main guns.

Additionally, experience with existing ships had revealed that the secondary battery, mounted in casemates in the forecastle deck, were effectively unusable in rough sea conditions.

Daniels approved these changes on 2 July, and the following month, BuOrd suggested a newly developed 50-caliber version of 14-inch gun be used, which Watt authorized in September.

The longer gun produced a higher muzzle velocity, which allowed the shells to penetrate an additional 2 in (51 mm) of armor at 10,000 yd (9,100 m).

New Mexico's turbines were used to power electrical motors that drove the screw propellers; her system was rated for 27,500 shaft horsepower (20,500 kW).

[7] The ships were armed with a main battery of twelve 14-inch (356 mm)/50 caliber Mark IV guns in four, three-gun turrets on the centerline, placed in two superfiring pairs forward and aft of the superstructure.

After extensive testing, it was eventually discovered to have been caused by overly lengthy chambers, which allowed a gap between the shell and the propellant charges.

[11][12] The guns were supplied with 100 shells each and they were capable of a rate of fire of about one salvo per minute, as demonstrated by extensive testing with Idaho in October 1942.

[13] The secondary battery consisted of fourteen 5-inch (127 mm)/51 caliber guns mounted in individual casemates clustered in the superstructure amidships.

[11][16] In addition to their gun armament, the New Mexico-class ships were also fitted with two 21-inch (533 mm) torpedo tubes, mounted submerged in the hull, one on each broadside.

Each tube was supplied with a total of six torpedoes,[17] which were Bliss-Leavitt weapons of the Mark VII type; these carried a 321 lb (146 kg) warhead and had a range of 12,500 yd (11,400 m) at a speed of 27 kn (50 km/h; 31 mph).

A second torpedo bulkhead was installed outboard of the original one, and all three ships received anti-torpedo bulges to further enhance their resistance to underwater attack.

The ships' armament was also revised, with the main battery turrets being modified to allow elevation to 30 degrees, greatly extending the range of the guns.

Since many of the American battleships had been sunk or damaged at Pearl Harbor, the three New Mexicos were too valuable to be removed from operational status for the necessary amount of time to substantially rebuild them.

Throughout this period, the fleet was based in San Pedro, California, but it frequently operated in Hawaiian waters, off the coast of Central and South America, and in the Caribbean Sea.

[30][31][32][34] In mid-1940, the Battle Fleet was transferred from California to Hawaii in response to rising tensions between the United States and Japan over the latter's aggression in the Second Sino-Japanese War.

By 1941, President Franklin D. Roosevelt initiated the Neutrality Patrols to protect American shipping caught in the Battle of the Atlantic during World War II.

After arriving in California in January 1942, the ships spent the next several months patrolling the west coast of the United States and escorting convoys in the area.

These operations continued into January 1944, at which point the fleet began preparations for the next major campaign, which would be directed against the Marianas Islands in the central Pacific.

By September, Mississippi had returned to the fleet to relieve New Mexico for her own refit, and she and Idaho shelled the island of Pelelieu, the last battle of the Marianas campaign.

The fleet thereafter withdrew to Manus Island to prepare for the coming invasion of the Philippines, while New Mexico departed for a refit at the Puget Sound Navy Yard.

[30][31][36] By January 1945, New Mexico had rejoined the fleet, the next target of which was the invasion of Lingayen Gulf on the island of Luzon; she and Mississippi supported the amphibious assault and during the battle both ships were struck by a kamikaze suicide plane.

Mississippi survived the postwar draw-down in naval strength through conversion into a gunnery training and evaluation ship to replace the older battleship Wyoming.

With the new hull number EAG-128 to designate her status as an auxiliary vessel, she was reconstructed at the Norfolk Naval Shipyard from November 1945 through April 1948 with a new armament that included a variety of different weapons, thereafter serving as the flagship of the Operational Test and Evaluation Force.