Although in the UK one often finds ordinary horizontal and horizontal/vertical millers badged as being by Bridgeport, these were actually made by Adcock and Shipley, a Leicester-based company owned at one time by Bridgeport and responsible for producing copies of the "real American Bridgeport turret miller" (the Serial number on these models should be on a plate inside the column door - the last 4 digits being the date of manufacture).  Adcock and Shipley also manufactured the later CNC "Interact" millers and APC and BPC machining centres. Although the Bridgeport factory on the corner of Lindley Street and Capitol Avenue, Bridgeport in Connecticut, was demolished in 2010, spares are still available and will continue to be so for the foreseeable future.
Today the original form of the Bridgeport is, to distinguish it from later computer-controlled and larger models, referred to as the "Series 1". It was first manufactured in 1938, by two immigrant Swedish engineers Magnus Wahlstrom, a toolmaker, and Rudolf F. Bannow (died 1962) a pattern maker and radio ham. The two men had first met in 1928, when Wahlstrom had called to buy wooden loudspeaker parts from Bannow, then president of the "Bridgeport Pattern and Model Works". By 1929 they were in business together and attempting to develop an electrically-powered hedge clipper; however, when this idea was abandoned they began work on a vertical-milling attachment designed to fasten to almost any of the hundreds of thousands of plain horizontal millers then in use. This accessory, the first of many Bridgeport heads and their accessories, become known as the "Master Milling Attachment", and was originally designated by the partners as their "Model C". Smartly presented, with a polished aluminium housing, it was well engineered and equipped with a heat-treated and ground spindle running in four precision, pre-loaded bearings (with those at the pulley end allowed to "float" to accommodate expansion and contraction of the spindle) and with a quill that accepted (as standard) 3-inch long B-3 collets. The head was powered by a 1/4 hp motor that gave, in conjunction with 6-step aluminium pulleys, speeds of 465, 675, 1000, 1500, 2140 and 4250 rpm. The first production example was delivered in 1932 to the Atlas Tool Company of Bridgeport and the head continued in production long enough to become known as the "Model M" when eventually fitted, some years later, to the first Bridgeport milling machine. The partners, however, were not alone in the conversion market and by the 1940s many similar attachments were available, the best-known of which were the "Tree" from Racine; the Marvin from Michigan; the "Halco" from Detroit and Kearney & Trecker with their Dalrae-manufactured "Midgetmill" and "Speedmill" units.
One special version of the Bridgeport Model C was made for use on precision Hardinge horizontal millers (early versions were badged Cataract MD5) and these heads can be recognised by a Serial numbers beginning with the letter "H". Although identical (at a glance) to the standard Model C they were not as long and used a shorter "2VB" Collet closer. From the centre of the mounting holes to the bottom of the 2VB collet closer was 5 3/16" whilst the same dimension on the B-3 collet closer model was 611/16". 
By 1936 the original head been improved by the addition of a 1/2 hp motor, the inclusion of a sliding quill with 3.5" of travel - which of course radically improved the unit's drilling and boring performance - a choice of either a No. 2 Morse or a No. 7 Brown & Sharpe taper and an improved speed range that ran through 275, 425, 700, 1050, 2100 and 4250 rpm. With this much more useful specification business began to pick up and, by 1938, with sales of 500 units per annum, and seeking further expansion, the partners decided to incorporated the head in a completely new design of highly-versatile milling machine that they would produce themselves - the Bridgeport. Apocryphal, no doubt (but with the ring of truth) the initial sketches were said to have been drawn by Rudy Bannow on a paper bag whilst waiting to unload patterns at a customer's works. In 1938 the Company changed its name to "Bridgeport Machine Inc." and in August of that year the first Bridgeport turret milling machine was completed and shipped to the plant of the Precision Die Casting Company of Syracuse, New York - this first example, invoiced at $995, was later recovered by Bridgeport and donated to the Henry Ford Museum at Dearborn, Michigan. Today it can be found at the Precision Museum in Windsor, Vermont, U.S.A.
Because the Bridgeport was extremely well thought out its versatility was quickly appreciated in both toolrooms and production plants; not only did the whole upper turret pivot on the main column, but the round horizontal ram (driven by a rack and pinion on some but not all versions) could be moved forwards and backwards over a distance of 12 inches; once that setting had been achieved the whole assembly could then, if needed, be swung left and right and the head tilted over under the control of a large worm-and-wheel gear - the latter arrangement ensuring alterations could be made quickly and with great precision. The value of this latter feature can only be truly appreciated if you have ever have struggled to hold 75 lbs of cast iron in one hand - over at an angle in perfect alignment with an engraved mark - whilst simultaneously trying to tighten a miserably-small nut buried in some dark and oily recess hidden away at the back of the machine. Probably due to difficulties with rigidity, not all models had a clevis on the vertical-head end of the ram, later types being fitted with a more robust flat surface but retaining a clevis at the other end to mount a slotting attachment. These early models had a 5-inch diameter round horizontal ram, distinctly different to the "square" type that was available from September 15th, 1952; this later ram assembly, and its mounting, could be used on earlier machines and was advertised for a time by the factory as a conversion unit. The new design had so improved the rigidity of the ram that it allowed the reintroduction of a clevis end for the vertical head (like that the competing Cross miller) and so, once again, it could be "nodded" backwards and forwards through an angle of 45 degrees each side of vertical. As an interesting aside some early (round-ram) machines have been found with a Type C head having a much-improved quill travel of 5 inches, rather than the usual 3.5. As a note of caution, when milling, ensure that the quill is fully retracted and locked; the sideways forces involved under all but the most gentle of cuts can be sufficient to distort, permanently, the quill housing. On light jobs, using the first inch or so of quill travel to apply extra depth of cut is acceptable - but, do not over do it.
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The classic Bridgeport Series 1- a 1966 "Model 9BRJ" and "Model 12BRJ" - so called because they were fitted with a table either 9 or 12 inches wide and a 1-H.P Model J Head (the Series 1 was listed for the UK market with tables lengths of 32, 36, 42 and 48 inches with, respectively, 20, 24, 30, and 36 inches of longitudinal travel; all the tables were 9 inches wide and had 9 inches of cross travel on the 9BRM, and 12 inches on the 12BRM - the only difference between the two Models.) As an option, hard-chrome plated ways and gib strips were available, the process being applied to the top of the cross-slide longitudinal movement, the top of knee-cross movement, back of knee-vertical movement, column to knee gib and cross slide to table gib.
A detailed Data Pack, Manual and Parts List can be ordered here

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Another vital ingredient in the machine's success was the location of the motor to one side of the spindle, with drive by a V-belt; this meant that both the fine and drilling feeds of the quill could operate along the axis of the tool, no matter what its angle - and in a series of machining and drilling operations, which required different combinations of quick-action and fine feed movements of the quill, all could be carried out without having to reset the head. A further advantage of the side-drive motor was that the quill was left clear for a draw bar to pass through and retain cutters or their holders - some millers of this type, while having the same quill-feed arrangements, employed a motor fastened to (and so blanking off) the top of the spindle housing - so forcing the employment of awkward-to-use screwed retaining rings on their noses (a further consequence often being the need to use expensive custom or at least non-standard or modified cutter holders). Details of the various milling, drilling and boring heads can be found here.
Early Bridgeport millers had a relatively small tables, those listed being 30" x 9", 30" x 12" and 36" x 12". One advantage of a short-table model - especially a well-used one, is that wear across the central section - the most-used part - is better spread out when compared to the much heavier, longer versions. Later tables were offered (in the UK) with lengths of 32, 36, 42 and 48 inches with, respectively, 20, 24, 30, and 36 inches of longitudinal travel. As an option, hard-chrome plated ways and gib strips were available, the process being applied to the tops of the cross-slide and knee, the back of the knee and the column and cross-slide-to-table gibs.
On March 18th, 1954 the 20,000^th machine left the Bridgeport factory (a building newly-erected two years earlier) bound for the Pioneer Electric Research Corporation of Forest Park, Illinois. Despite one machine being produced every 45 minutes, such was the demand for machine tools in the early 1950s that a sixteen-month backlog of orders, totalling over 3,600 machines, was not an unusual position for Bridgeport to be in. Advertising expenses for 1953 of less than $12,000 point to how the millers virtually sold themselves - and it was not uncommon in such an active market for "little-used" examples to fetch three times the manufacturer's list price.
Although for many years Bridgeport produced only the "Series 1" (though a simple horizontal miller was produced for in-house use) it was eventually converted into many special-purpose examples, both by the factory and third parties: machines with 2D and 3D hydraulic copying in both manual and automatic versions; T-shaped heads to allow a single head to be moved sideways, or up to three heads to be mounted side by side; automatic copying and precision tracing machines and, by the early 1960s, the Moog Hydra-point three-axis, numerically-controlled miller that was manufactured in England by Moog Hydra-point Ltd. on a machine made by the Bridgeport subsidiary, Adcock & Shipley Ltd. of Leicester - who had first built the Series 1 model under licence in 1959 (the initial model having a 42" x 9" table). One version, the "Series 1 CNC" was heavily modified for CNC control as a "toe-in-the-water exercise; details can be found here.
By 1963 more than 60,000 examples had been manufactured, with plants in Bridgeport Connecticut, USA, Leicester and Bridlington in England - and Singapore. It was also a widely copied machine, with dozens of companies in Taiwan - and even fourteen in Spain at one time - turning out examples of greatly varying quality and performance. Today Bridgeport makes not just millers but turning and grinding machines as well, a sale being made to Rolls Royce in 2006 of a large and complex grinding machine on which to finish jet-engine parts.
As the years went by demand for the machine changed and costs came under increasing scrutiny; under various ownerships production was shifted around the globe and when owned for a time, by Textron, the machines for the USA market had their main castings poured in India and then shipped to England where the column, knee and table were built up. The part-finished millers were then sent to America where Singapore-manufactured heads were fitted and the machine put through final assembly and painting (at least, that's the story from an ex-employee.…). Even so, one would have thought that a single plant could have achieved cost savings equal to or even better than those involved in packing up and shipping components half-way around the world and back again.
After a series of ownerships, management buy-outs, amalgamations and trading, Bridgeport eventually became part of the Goldman Industrial Group together with other leading machine tool companies including Bryant Grinder, Fellows, Hill-Loma, J. & L. Metrology, and Jones & Lamson. In 2002 (at machine Serial Number BR-247388) Hardinge took over the rights to Bridgeport's knee-mill designs (on a 7-year arrangement) and then, in November 2004, acquired full rights to the Bridgeport name to join Hardinge-branded high-precision lathes, Kellenberger cylindrical grinding machines, Hauser jig grinders, Tschudin high-production cylindrical grinders, Tripet internal-grinding machines and Hardinge-branded collets, chucks and indexing fixtures. In the UK the Bridgeport division, formerly based around Adcock & Shipley in Leicester, continued in operation - but not the Bridgeport manufacturing operation which, by November 2004, was in receivership. A total of $7,250,000 was paid by Hardinge for the acquisition but this included finished goods (including CNC machining centres) worth an estimated $4,100,000 (a very late-edition of the Series 1 catalogue can be found here). The continued use of the Bridgeport name seems assured with Hardinge currently having over 800,000 square feet of manufacturing capacity in its operations in England, the United States, Switzerland, Taiwan and China..
An interesting article about manufacturing the Bridgeport can be found reproduced from the American Machinist Magazine for November 2000..

Bridgeport Series 1 "9BRM" and "12BRM" 0.5 hp. turret miller designed as a "medium-light duty" machine. The model was identical to the BRJ but fitted with the lighter Model M Head.